My prediction is this. Vanitas will be back in KH3 and for a 1/4 of the game will follow MX and be a major nemesis for Sora. However at the halfway point of the game Vanitas is going to get the X Blade and will kill MX with it proclaiming himself as the master. By the end of the game, the final battle will be between Vanitas and Sora for the fate of everything and everyone.
In the end, only one will live.
In the end, only one will live.