The only Vita game I have right now is Persona 4 Golden, but that one alone was definitely worth it: Persona 4 is my favorite RPG (and my second-favorite game ever), and almost all of the new content is awesome.
Though I also plan to get Ragnarok Odyssey, Gravity Rush, Disgaea 3: Absence of Detention, LittleBigPlanet PS Vita, Dragon's Crown (when it comes out), Mortal Kombat, Soul Sacrifice (when it comes out), Tales of Innocence/Hearts R (if/when they come out in the US), and Zero Escape: Virtue's Last Reward, since they all look great.
Well, in the meantime, I have a New Game+ in Persona 4 Golden, and my old (digital) PSP games and PSOne Classics to play on it.