I think that Vanitas is the Xehanort that was Ansem the Wise's apprentice. I mean it fits, right. "Xehanort" appears out of nowhere after the events of KH:BBS, and all he remembers is "his" name. Master Xehanort is (at some point) saying "defeat the darkness" and "correct his mistake" while gesturing toward Vanitas. This could mean that MX "defeated the darkness" and "corrected his mistake" by destroying Vanitas' memory. MX probably thought that this would destroy the darkness within Vanitas, even though ones true nature can never be changed (like when Sora became a heartless). Vanitas might have then destroyed MX (or destroying Vanitas' memory took too much of MX's power) while keeping the memory of the name "Xehanort".
