Soul Sacrifice Delta, if you like games with fucking mountains of content. I haven't got Delta yet but the original had a ton of content and massive amounts of free DLC.
Muramasa Rebirth, if you haven't played it on Wii. Well even if you have, there's some cool DLC for it. You can be a cat girl. (i've beat the cat girl DLC like 100% and barely touched the main stories haha)
Zero Escape: Virtue's Last Reward, if you haven't already beat it on 3DS. Though it's superior on vita regardless.
Ragnarok Odyssey Ace, if you like action games or ragnarok online references
Tales of Hearts R, if you like Tales games. It's fantastic, i'm playing it right now.
Sorcery Saga, it's the most adorable game on the vita. So goddamn kawaii.
Probably way more but I ran out of time to think of more. The vita has a ton of great games.