Alright, so I've searched the forums and haven't found anything like this, but I do apologize if it has been posted here before. I know the topic has come up before about why Vanitas was shown beside Young Xehanort in DDD as well as where he may be now, but I think I may have an idea/theory to answer this.
Ok, so we know from Nomura in one of his interviews he states that Vanitas showed up because it was Ventus' heart reacting inside of Sora. Now, I still see some confusion here from other fans, so I just want to clear one thing up. Sora did NOT see Vanitas, only Ventus did (and us as the audience, of course). What Sora saw and heard was YX saying, "Hypocrite, you are the one that has made your heart a prison... Even if you are not the prisoner." Ventus (from what we can gather) saw and heard the same thing but with Vanitas saying it instead. That is why after it was all said and done, Sora asks, "My heart's a prison? :sad:" instead of questioning why there was another boy standing beside YX that looked exactly like him. He didn't see Vanitas; he saw YX and that was it.
Now continuing from this, I think there may be a double meaning behind the line "Even if you are not the prisoner." YX said it to Sora and was talking about Ventus being the 'prisoner' inside his heart. But what if the same could be said about Vanitas? What if Vanitas said the same thing because it was also true that HE himself was also a prisoner inside of Ven's heart? Looking back at the end of BBS with the resolution of Ven and Vanitas' fight within their dive to the heart, Vanitas fell into his pillar. Now, I was with a lot of people in first taking that as a way of saying the two finally joined back together, but obviously that's not the case since Ventus went into a state of suspended animation like a Princess of Heart would when they lose their hearts. Meaning, his heart still only consists of light. Going off of this, could it be said that Vanitas fell into Ven's heart and got stuck there still as a separate entity and became his 'prisoner'?
If this is the case, then Vanitas is trapped inside of Ventus' heart which is trapped inside of Sora's heart. And this kind of matches up with what YX said from the newest trailer featuring the Toy Story world. YX states, "There is a darkness we are missing and must reclaim." Now, it's really just an opinion here, but I think he's talking about Vanitas-which is why I said it kind of matches up. If he is talking about Vanitas, then them [the seekers] trying to find clues as to how hearts connect... this could be why: If they want Vanitas, they'd first have to get Ventus out of Sora's heart and then get Vanitas out of Ventus' heart. That's some crazy heart extraction going on there.
Anyway, that's pretty much all I have to offer. Let me know what you guys think.