So, I've got this idea bouncing around in my head. Not a fanfic because I doubt I ever actually write it. Still, gotta get something down, right? It goes like this:
First things first: my theory of Vanitas is after he 'died' he found his way into Xehanort's heart and thus to Ansem's. This explains why, after Riku was posessed by Xehanort's heartless he began exhibiting an appearence, fighting style and even some behavior similar to Vanitas's. It also explains Vanitas's appearence in Dream Drop behind Young Xehanort. Sorta... Any way, that's how I explain the connection between Riku and Vanitas. Now, Riku pretty much rid Ansem from his heart in KH 3D, so for the sake of the story, Ansem left Vanitas behind. And without furthur ado, a summary of the idea bouncing around my head.
After becoming a Keyblade Master, Riku believes himself unworthy still because of the darkness in his heart. Yen Sid suggests Riku confront this darkness by Diving into his heart. When he gets there, he discovers -in full masked attire- Vanitas, who is fading, but unwilling to let go of his life. Desperate, he fights Riku in an attempt to posess him and live again. Riku defeats Vanitas but Vanitas does not die. Instead he gets his own body back (the one created when he and Ventus were split apart), don't ask me how, I haven't gotten the logistics down. Vanitas responds to this new situation predictably -with open hostility. Though weak, Vanitas rough and ready to take on Riku, Sora, Kairi, Lea, Mickey and Yen Sid. However the old wizard binds the boy and lets him know that violence would not be tolerated. Yen Sid then explains what he knows of Vanitas, but it interrupted by an Unversed which is proptly cut down by Sora. Vanitas screams in pain -and confusion since he believed he had learned to cut himself off from the creatures. Sora, horrified tries to help, only to be pushed back by another, larger Unversed.