It's a nice theory, and I wouldn't be surprised if it turned out to be true in one way or another. However, something about this really hurts it for me. It doesn't kill it, as I can think of a billion work arounds, but it makes it seem unlikely.
Let's say Ven's heart is not unlike Kairi's; that it hid itself behind Sora's heart for some reason or another. When Sora stabbed himself with the Princess keyblade, only two hearts were released: His, and Kairi's. Why would Ven's heart not make an appearance when Kairi's heart, which is doing essentially the same thing, did?
I thought of that.
If Ven's Heart went to Roxas, there must've been a reason. I don't know that reason, but for that reason, I think that Ven's Heart stayed in Roxas's --Sora's-- body, and wasn't released along with Sora's and Kairi's hearts. Then, when Sora's body woke up in Twilight Town with Ven's Heart, he was Roxas.
...that makes sense, right?
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Now, an alternate theory I've been playing with is that when Sora lost his heart and cast his body away, it happened to be near Ven's Heart, which would've been floating in suspension if Ven's body was "erased." It's less likely, and I don't fully believe in this theory, but it's another possibility.

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