Now I really want that game in Final Mix. I don't remember very well... but didn't Aqua, after fight with that monster, found a Castle in the middle of the Realm of Darkness? I don't remember if I saw this in cut scene or in a photo, but I'm sure she found something; What was that? I don't recall know the story after that...
Well, probably just like Axel and the others come to be somebody, Vanitas come back; he was able to come back to young again. Everyone who vanish is there, so he come back with a "new form", more powerful, that's the why he have this young appearance. That's my guess...
