I want to say a Vita as PS1 support will be coming quite soon but the problem is that it doesn't support all of the PSP games in the store yet.
http://www.giantbomb.com/news/initial-slate-of-vita-compatible-psp-games-revealed/3975/
This is an old list of 275 games that worked early on. They've been adding more. I know this because I've only recently(about 2 weeks ago) been able to get Killzone Liberation and Undead Knights on the system. They were added to the Vita store fairly recently and they're not on that list. I've owned both long before the vita released, btw.
I somewhat worry about there being a need for a similar list for when the Vita gets PS1 support.
If you can get an updated list of the PSP games available or perhaps list some games you're interested in and someone here with a Vita like myself can check to see if they're available for you that might help.
Other wise, I'd say go with the PSPGo. it seems to fit your needs. Especially when it comes to portability. The Vita is the least portable.
4 ≈ One
