As of right now, all we can really conclude about Vanitas is that he has a gold/yellow eye and dark eyebrows. His identity is still a mystery. So, here is my theory.
I personally think he has a much greater chance of becoming Xehanort than Terra does. Everything would fit together this way.
Why? A few reasons. First, notice the strikingly similar features he shares with Riku. Their outfits are similar, they have the same fighting stance, the blue gems on Riku's and Vanitas' keyblade are the same, and in the most recent trailer, Vanitas reaches out to Ven in the same way Riku does to Sora.
How does this make Vanitas Xehanort? Well, if Vanitas becomes Xehanort, then that means he is also Xemnas and Heartless Ansem. Since Heartless Ansem overtook Riku and possessed him, that would explain why Riku gets a similar outfit to Vanitas, and why Riku's keyblade has the same blue gem as Vanitas', etc.
I personally think he has a much greater chance of becoming Xehanort than Terra does. Everything would fit together this way.
Why? A few reasons. First, notice the strikingly similar features he shares with Riku. Their outfits are similar, they have the same fighting stance, the blue gems on Riku's and Vanitas' keyblade are the same, and in the most recent trailer, Vanitas reaches out to Ven in the same way Riku does to Sora.
How does this make Vanitas Xehanort? Well, if Vanitas becomes Xehanort, then that means he is also Xemnas and Heartless Ansem. Since Heartless Ansem overtook Riku and possessed him, that would explain why Riku gets a similar outfit to Vanitas, and why Riku's keyblade has the same blue gem as Vanitas', etc.