To be honest, I have no clue, that's why it's not a theory.
Originally, I was going to post a theory where Vanitas is like Xion.
To sum it up, Vanitas looks like Ven with black hair (and gold eyes), but is actually a replica of Terra. In other words, he, like Xion, has taken on the form of the original's most important memory (in this case, Ven).
It still holds some merit, given that Xion doesn't have the same exact hair style as Kairi, and neither does Vanitas compared to Ven, but I discovered that Sora's hair matches perfectly and couldn't let that go, especially since some have said they have the same VA.
If it's true, it's a total mindfuck and I'm at a loss for the moment.