There are Vita Dev Kits but you don't need them anymore.
You can now publish your Unity game using the free Unity/PSM download, the free PSM app from the PlayStation store and a retail Vita.
Just some quick tips:
Instantiate/Create is a system killer. If you create game objects in game, your Vita will stutter and slowdown very quickly. You'll need a pooling system and to pre-load quite a lot of stuff before the game/level starts.
Overdraw is usually the most common issue on PSVita / PSM due to the hardware. Try to make sure you're not drawing a ton of stuff on top of one another.