So recently I've been playing some Dream Drop Distance. And I've come to a conclusion that Vanitas is probably Xehanort.
I've noticed that Xehanort, his younger self and Vanitas all share the same shade of orange eyes.
Also, Young Xehanort has the ability to time travel, so maybe he took on the form of Vanitas, created the Unversed and sided with his older version.
That also brings me onto a small point, the Unversed and Nightmare logo are slightly similar.
But, my main point is that in the final scene of Sora's story in La City, you'll see Young Xehanort, and when he says 'Even if you are not the prisoner', he's pictured with a hallucination of Vanitas next to him. The reason why you saw Vanitas is because Ventus, who' inside Sora's heart, was able to easily show that Young Xehanort was Vanitas, thanks to them sharing the same heart.
Also, another bit of evidence is that Vanitas clearly has ties to the darkness and is one of the 13 because of his eye colour, but he wasn't on a chair with the others in the scene where Xehanort attempts to take Sora as a vessel. Or, he kinda was, only he was in the form of Young Xehanort instead of Vanitas.
And there's my theory, please don't judge me if I got something wrong or if it's already been confirmed, but you can kindly tell me what I need to correct and what more evidence there is.