Re: what, exactly, happened to vanitas?
I don't think their hearts stayed merged, especially because by destroying the X-blade Ven shattered his own heart. I also think that Ventus and Vanitas had grown so independent of each other, that I can't really consider them one heart but two separate hearts.
Their hearts never merged fully again, as Ventus
rejected the fusion of their hearts.
Many people seem to forget that their fusion was never properly finished.
Beside that, heart fragments can grow into full hearts if they are nurtured properly and Ventus' heart
was nurtured properly in the four-year-gap between Vanitas' initial separation and the start of BBS proper.
Nomura explicitly stated this in the BBS Ultimania that Ven's heart was not fully formed (due to the damage done when Vanitas was removed) and that's why he had blank eyes when Xehanort first brought him to LoD.
They
are two separate hearts as they grew during the four-year gap until BBS started.
i disagree. i think that they stayed merged, and thats WHY the destruction of the x-blade put ven in a coma. if they didnt stay merged, the destruction would have just ripped him in two again and hed go back to zombie ven mode like when MX brought him to eraqus.
No, Vanitas and Ventus never merged fully, as Ventus resisted the fusion to a degree, that was why they even fought in the awakening platform in the first place. Vanitas wanted to complete the merge/fusion while Ven wanted to completely undo it.
The destruction of the proto-X-blade caused Ven to fall into a coma because it totally shattered his heart. That he was "Zombie-Ven" when he was brought to Eraqus was because of the interjection of Sora's heart who helped him to awake, but his heart wasn't yet fully healed.