PSN Games are split into three categories: Vita Download - You can download it from your Vita. PS3 Transfer - You can download from a PS3 and transfer to the Vita. Vita Incompatible - Games that can't be played on the Vita.
My rough estimate of games in each category (pure guess): UMD Only - 10% Notables games include Crisis Core. Birth by Sleep, Valkyrie Profile Vita Incompatible - 5% There don't seem to be too many of these, or at least that people get upset about PS3 Transfer - 10-15% Quite a few games and most new releases start as transfer only Vita Download 70-75% The majority of the games on PSN
So yeah you can play most games especially if you have a PS3. --- "That was pretty interesting. But dropping a warship on me is cheating... Take it back!" - Xenogears