Before Master Xehanort (MX for short) extracted Vanitas from him, Ventus' original personality was very gentle, weak and lack of self-confidence. he didn't fight back when MX summoned the Neo-shadow Heartless to surround him, it was not because he didn't want to use the power of darkness inside him, it was because he was too scared and thought very pessimistically that there was no way he could fight those monsters, so he just gave up there. Well, the kid was only 11 or 12 back then if you assume he was like 15 or 16 in the BbS game. The whole 'extraction' event happened 4 years earlier than BbS game, it was also when Sora was just born. Ventus was actually an orphan who had never seen his parents.
In general, Ven was a very timid, diffidence, weak boy. I seriously wonder how such a weak kid like him could wield a keyblade after all, since it requires a strong heart to wield a keyblade according to the original game setting. Well, maybe it's because MX raised him and passed the keyblade to him. Anyway, you all know from the game that after the Heartless attack, he fell unconscious and then MX extracted Vanitas from him. The chapter that describes this event and what happened after Vanitas' birth is called Dark Memory.
The newborn Vanitas, I'd say, is very much Ventus himself. I don't know how to put in words precisely, it's just like, y'know, when you see someone flip between their personas, you still kinda know they are the same person, that kind of feeling? The newborn Vanitas inherited Ven's feelings and thoughts directly, like he continued what Ventus was thinking previously but now with a much darker attitude and way of thinking. He still "remembered" the fear Ventus was feeling when attacked by countless Heartless. But he successfully turned this fear into power as he was able to summon his Keyblade right after his birth. (When MX summoned all the heartless, MX told Ventus to turn fear into power to defeat them, Ventus couldn't accomplish that, but Vanitas did it successfully.)
When Vanitas first saw Ventus lying on the ground, he felt somehow sad (maybe due to the separation), but soon his feeling towards Ventus turned into contempt and hate, calling Ventus an "empty shell", like Ventus was his weakness, and he hated being so weak. Or, you can say Ventus hated himself being so weak. He wanted to be strong, and that's how Vanitas' personality came to exist. I know in the game, the two of them are so different and basically no one see Ventus and Vanitas as the same person. But in the novel, at least in the first part, you can still feel Vanitas is indeed Ventus himself. Only later, Vanitas become more and more different from his original self, a.k.a. Ventus.
Right after his birth, Vanitas was very aware of his own being, that he was the personified darkness and negativity of Ventus. But then he started to question why he was made born. Every time he "felt" a thing, an Unversed would be born from the dark aura emitted from his body. But when Vanitas first saw the little monster, he felt irritated by the fact that such monster was born from him. So he destroyed the Unversed with his keyblade without any hesitation. But the Unversed was literally like a part of his own body, his extension, so every time he destroyed an Unversed, he himself would feel strong PHYSICAL pain running through his body. (Does that make you NOT want to kill any Unversed in the game anymore? At least I can't play the game anymore after knowing that how much pain Vani would go through every time the three keyblade heroes killed an Unversed T_T)
And after the Unversed was destroyed, the pain and negative feeling that the Unversed experienced during its death would return to Vanitas' mind, which added more to the irritation Vanitas was feeling and in turn another Unversed of "irritation" would born from him. It looks like at the time Vanitas was just born, he had no self-controls on the creation of Unversed, but rather Unversed were born spontaneously every time he had a strong emotion rise. But later in the story, he somehow trained himself to freely create Unversed according his own will (sounds like something like "tame your own emotion" sorta of thing xD).
At the time Vanitas was just born, he did not have a human face. He was basically like a humanoid Heartless, with pure black skins but with red eyes instead of the yellow eyes that Heartless possess. If you have noticed from the game, all Unversed have red eyes, so he was just like an Unversed himself at the time. But then MX took the unconscious Ven to the Destiny Islands, planning to leave him there to rest in peace. And then the newborn heart of Sora's came in contact with Ven's fractured heart as you see in the game. Surprisingly, that time Sora did not only touch Ven's heart, he also directly contacted Vanitas' heart. After feeling a sharp pain in his chest, Vanitas' body fell to the ground of the wasteland near Keyblade Graveyard and almost lost his consciousness, feeling being engulfed by absolute Darkness. That was when Vanitas heard Sora's voice and then he saw Light. The Light eventually surrounded his whole body. Then he heard the voice saying "Let's open the door" (if you remember from the game, the line was voiced by both HJO and Jesse McCartney at the same time, and now you can say it could be Vanitas' voice instead of Sora's). Vanitas then opened his eyes and found out that now he had a normal boy's face. He also realized that someone, probably also a newborn heart like himself, had saved Ven's heart from completely fading away.
So after being saved by Sora, Ventus was able to walk around, but he was still unconscious, he was just mindlessly walking around, like a zombie. MX brought him back to the Keyblade Graveyard and rejoined with Vanitas. But as soon as Vanitas saw Ventus, he couldn't suppress the feeling of hatred he has towards this "empty shell". He literally kicked Ventus till the blonde fell onto the ground, groaning in pain. And looking at Ventus whimpering like that only led his hatred to grow even stronger to the point he summoned his Keyblade wanting to destroy Ventus completely, which of course was stopped by MX. MX talked about his need of Ventus for his great purpose so he would not allow Vanitas to kill the boy. Unlike Ventus, who actually respected and looked up MX as a master, Vanitas indeed disliked the geezer and had no plan of being used by him for any scheme. But he was his creator after all so he didn't defy the old man openly. But then he started wondering why he was created at first place, what's the meaning of his life, that kind of things.
Because Vanitas told MX that if MX kept both him and Ventus, he couldn't guarantee that he could keep Ventus alive, since he found it was hard to suppress his impulse to hurt Ventus (yeah, you little sadistic bastard xD). Therefore, MX had to take Ventus to the Land of Departure, asking Eraqus to take care of the boy.
But then after Ventus left, Vanitas started to miss him, he kept wondering why they got separated from each other. It made him feel so miserable and painful. At the same time, he also felt very lost and anxious about his future, without Ventus, what would become of him? What was he supposed to do? What's the meaning of his existence? Why did he feel so suffocated in his chest? The more he thought about these questions, the more anxious, irritated and lonely he felt.
Quite unfortunate for him, as I mentioned above, every time he felt an emotion, it created an Unversed. Looking at the Unversed was like looking at his own weakness that he did not want to admit, which made him feel very unhappy and wanting to kill. So Vanitas started to keep killing the Unversed with his own hands, which, as you can imagine, only brought more physical pain and emotional agony to himself as the defeated Unversed return all the pain and negativity back to him, which in turn spored more Unversed to born. Being the incarnation of "negative emotion", the more negative emotions Vanitas received and felt, the stronger he would become. And he wanted to become strong because he hated being so weak, killing the Unversed was the only way he knew at the moment to get stronger, plus killing the Unversed could bring a momentary relief even though it only felt worse after he killed them. So he never stopped the killing.
It's like a vicious cycle that Vanitas could not escape from, he wanted to be strong, he wanted to get rid of the Unversed which were the symbols of his weakness and their unpleasant existence caused him feel very unhappy. But by getting rid of them, he only felt more pain and negativity. And in turn he started to hate Ventus even more, because he envied Ventus. The reason why he envied him? While the unbearable pain was crashing him, torturing him, smothering him, Ventus did not feel anything (Ventus was still in his Zombie status). It was a contradicting feeling. Vanitas wanted to become Ventus, he wanted to become that person who could not feel a thing and who did not remember or know any bad memories that had happened/was happening to 'them'.
At the same time, MX only coldly watched Vanitas harming himself from far away. Vanitas was all by himself when going through this struggle. He desperately wanted to escape from this vicious cycle of pain and suffering. But as young as he was at that time, he simply didn't know any other ways other than killing the Unversed. Until the point the pain was too much to endure, he finally collapsed onto the ground of Keyblade Graveyard. And one thing you probably would never expect to happen, that Vanitas cried, tears running down his face silently as he lay on the ground, alone. Well, after all he was only a kid back then. While it's surprising because we almost never see Vani show any weakness (expect that one time after he was defeated by Ven in the end of the game), you couldn't say it's not reasonable for him to cry at that time. According to the novel, Vanitas is not as pure darkness as we think from the game. Nothing is 100% 'pure'. He could feel Ventus' heart, meaning he could feel good and positive things from Ventus. According to the novel, he very well understood what positive emotions like love, happiness and friendship are. That's what makes it even more tragic. Being a dark creature, he was, despite the common belief, craving for Light and salvation that could save him from this endless pain and suffering.
Then MX came to him, telling him the only way for him to achieve salvation was to become the X-blade. Becoming the X-blade was the only way for him to truly unite with Ventus again, and the X-blade would bring the salvation not only to him but also to the whole world. Of course, Vanitas didn't give a shit about the world, but he desperately wanted to free himself from this unbearable fate, thus to become the X-blade had now become his goal, his whole life purpose, and his only Light of Hope to lead him out this Darkness. Now you understand why he made 'that' expression at the end of the game when the X-blade was slipping away from his grasp, don't you?
Meanwhile Ventus at the Land of Departure was being taken very good care by Aqua, Terra and Master Eraqus. At first he was very much like a vegetable, or zombie, he didn't respond to any of the three Keyblade Wielders' questions or actions. But over time, he regained the ability to talk, to feel and to response. This chapter is called "Contrast", and this chapter literally made me cry.
The structure of this chapter is like this: first, the author describes how the four in the Land of Departure living like a loving family, how Ventus was loved by the other three caring family members. Then, the scene turned to the wasteland of Keyblade Graveyard, where Vanitas was all by himself, alone, feeling the warmth that Ventus' heart transmitted to him. Despite the momentary comforting feelings that briefly warmed up his lonely heart, his heart turned even colder once the warm feeling was gone.
Then it turned back to the Land of Departure again, telling Ventus' peaceful daily life with his 'family' and how he started to recover under Aqua and others' loving care. Then we see Vanitas again, beaten up by MX, who coldly told him that his training was not enough and he needed to get stronger. And then the old man left, leaving the badly injured boy all alone in the wasteland. At this point, Vanitas no longer cried any more as he understood too well that no one cared if he cried or not, he was all by himself. He had to get stronger, strong enough to accomplish X-blade, to reach his goal. Crying and whining wouldn't get him anywhere. He just kept practicing on fighting skills by killing the Unversed, embracing all the negativity and enduring all the pain from killing parts of himself. Even though sorrow and sadness was part of the negative emotions, but Vanitas knew these things wouldn't get him stronger, only Hatred and Loneliness was needed for his purpose of growing strong. He didn't need someone else to come to save him from this fate because there would be none, he could count on no one but himself, he was his own Saviour. Thus, he became more and more cold-hearted everyday, forgetting all the sorrow, remembering only the hatred. He now devoted himself to seeking the true power of Darkness. He no longer shed any tear, as the old tears had now already dried up.
Finally in the Land of Departure, Ventus fully recovered and became a lively boy again. Meanwhile at the wasteland of Keyblade Graveyard, as Vanitas felt how Ventus was chatting happily with his two friends, his heart was now completely filled with malicious hatred and darkness. He did not bother to figure out what it was that he hated most, maybe Ventus, maybe Ventus' friends, maybe MX who separated him from Ventus, maybe this unfair fate, or maybe he just hated this world as a whole. By the end of Chapter Contrast, this Vanitas, has become the Vanitas that we all see in the game, proud, cruel, sadistic, twisted, insane, and pure evil