This Theory could be true, But that would mean that since Vanitas is a keyblade user, Xehanort's heartless and Xemnas would also be Keyblade users. He may not remember that he was a Keyblade user, but Sora nor Roxas knew they were when they first found out. I think there is a connection with Riku and Vanitas. Their fighting stance are nearly the same, and also their weapons and clothing are very similar as well. There are too many unkowns to make any solid suggestions, but we need to look at similarities and make more responsible conclusions