The official site now has an extended trailer (with Tifa shown at the end) for those who are interested.

Also, although it doesn't confirm Vaan being in Dissidia Duodecim, I read somewhere that Vaan's voice actor (I think it was his English voice actor) said that ever since May 2010, he's been busy working on something with Square Enix, or something to that degree. Again, keep in mind that this doesn't confirm Vaan in Dissidia Duodecim.