I think the supposed parallel (or metaphor) between Roxas wanting his own individuality apart from Sora and Vanitas rejecting his individuality and wanting to reunite with Ventus, is a slight misinterpretation of Vanitas' character.
I disagree with the notion that Vanitas never wanted to be his own person. I’ve always thought he wanted to be his own person. And yeah, I know he wanted to reunite Ventus, but I thought this to be more in the sense that Vanitas would become a complete person while Ventus would cease to exist (rather than the other way around) His intentions are selfish for his own benefit. He didn't want to give his existence to Ven; rather, he wanted it to be complete, and the only person who might be able to do that is Ven.
In a hypothetical scenario: If Ventus wanted to reclaim his other half, Vanitas would cease to exist, and imo, Vanitas would want it to be the other way around. So, if the roles were reversed and Vanitas wanted to reclaim his other half instead, Ven would no longer exist.
— In other words, merging Ven's heart with his own was the only way he could become a "complete" person, and he doesn't care if Ventus ceases in the process. It would only be done for Vanitas' own sake, not for Ven’s.
That's how I interpreted it. After all, Vanitas is a selfish and boastful character. It never made sense to me to portray him as this miserable ol’ tragic figure who yearned to give up his existence for Ventus. The games depict the complete opposite.
It's also important to note that Vanitas only views Ventus as something he needs to reclaim. In his eyes, Ven is nothing more than his missing piece. He keeps referring to Ven as "the other half of me," as if he (Vanitas) were the original lmaooo.
“Join now with your better half”, or “I need the rest of me”
(The arrogance LOL. I don’t blame him, though.)
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In KH3 Re:Mind, Vanitas was bothered by the idea of giving up his existence for Ventus. He was well aware of the implications.
If he truly wanted to return to Ven, (which Ven offered to take him), why did he refuse? Even the mere thought of it bothered him.
Unless he was being sarcastic, which I highly doubt.
Because, even in BBS, when Vanitas took over Ven and they created the abomination Venitas, he tells Aqua and Mickey that he was not Ventus. Even after they had been merged into a "single" person, he still doesn't even refer to himself as "Ventus”. This doesn't sound like someone who desperately wants to give up his life for Ven, but rather someone who couldn't care less if Ven ceased. All that matters is that Vanitas can be a completed person if he absorbs Ven.
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Even in KH3, he still has “selfish” intentions. It never changed.
When Vanitas confronted Sora and his friends in Monstropolis, he goes on to explain that, while the children's negativity had helped to rebuild his heart, he still felt that his heart was incomplete. So, to resolve this issue, Vanitas wanted Ven's heart to join his heart in order for it to be "completed" (or to make him whole again).
— Which would be a huge advantage for him. Because, unlike in BBS, Vanitas does not need to take control of Ven's body and fight for dominance. He could simply extract Ven's heart from Sora and complete himself right there and then. Furthermore, Ventus would be unable to fight back because his heart was still sleeping, despite being stirred and reacting.
(Like lmao, why wouldn't Vanitas take advantage of that?) That was his one and only chance to become a complete person without any interference.
Overall
Vanitas never gave me the impression that he was rejecting his individuality; rather, he was more adamant about being his own being.
Vanitas' last conversation with Ven and Sora would have been meaningless if he didn't value his own individuality. The point of their conversation was to show that he wanted to be true to himself. While he acknowledged that Ven and Sora defined his existence, he chose not to change for them. He defined and accepted himself as “darkness” because that is how he chooses to live his own life or who he chooses to be.
He chose his identity over a chance at redemption.
Instead, I see some parallels between Vanitas and Roxas.
In my opinion, Repliku and Riku, are the ones who best fit the comparison between Roxas and Sora. Repliku never fully embraced his individuality, and he did not regard himself as his own being in the same way that Riku did.
