From what I stated in my "My Overall Theory" thread, Vanitas was destroyed, we know that. When something is destroyed in Nomura's concept, there is no death, there is only sleep. Vanitas is sleeping, the Young Xehanort at the end of the trailer we see is the real Xehanort. He has been sleeping, and since Vanitas originally was created from him, they are now in one body sleeping together. Yes, Xehanort and Vanitas are sleeping together. lol. They are waiting to be reborn since Xehanort became a Heartless. Now that his Heartless and Nobody are destroyed, they are now sleeping and he is awake. The reason I think he is still in the Dream Realm is because he is trying to fully separate him and Vanitas, so that he can bring back the Unversed and/or try to bring back his other forms.