PS Vita Isn't Ideal for Playing PSP Games
The PS Vita obviously has its strengths, and there are plenty of good reasons to own one. However, if you're thinking about getting a Vita mainly to play PSP games, I definitely don't think it's the best choice.
You know that huge difference between playing a game that was designed for a CRT TV on a modern display? That's almost what it feels like. I don't see many reviews talking about this, but PSP games running on the Vita unfortunately look noticeably worse to me, with much more visible aliasing. The resolution is doubled, but the visual characteristics of the original PSP screen just aren't replicated.
I recently bought a Vita hoping it could replace my PSP-3000, but I was pretty disappointed with the graphical result. Since I'm not particularly interested in the Vita's own game library, it just didn't make sense for me to keep the console.
If you're considering buying a PS Vita primarily to play PSP games, I definitely wouldn't recommend it. I'm curious to hear what others think, but personally, the graphical difference of PSP games running on the Vita bothered me quite a bit.