I don't remember where, but some Sony guy was talking about PS3 and PS Vita, and mentioned that the Vita is very powerful, but not identical to the PS3, it COULD ply most PS3 games, but games like Uncharted 3 and the upcoming The Last of Us push the PS3 to the limit, and would be among the list of games that would not run properly on PS Vita.
I believe that they could make it for Vita if they lowered the games visual quality constantly, until it was possible to run on the Vita, but for all we know Naughty Dog could be doing some crazy AI and gameplay things, that the Vita can't handle.
The Last of Us on Vita isn't out of the question, but I see it as unlikely.