If you are open to strategy RPGs, you can get disagea. There's also a ton of stuff in the ps classics section you can check out. I would recommend checking out the shop by rpg.
Dragon Fantasy Book 1/2 is reminiscent of old school Dragon Warrrior. Silent Hill Book of Memories is similar to Diablo and you can probably get that pretty cheap. Tactics Ogre and FF Tactics are 2 of the best SRPGs ever. Soul Sacrifice is a 3rd person action rpg similar to Monster Hunter. Most of the Final Fantasies. Most of the Personas. Crono cross Chaos Rings next week
Hit the shop, I cant remember all the games. There's quite a few RPGs if you are willing to play some indie type games.
haven't tried ps1 games on my vita yet. but the psp ones look ok. I suggest that when looking for games do it from vita as you can use the psn store to see some title then switch over to the web browser to find more information and use the youtube app to see some videos. being able to have multiple applications open on the vita is just a nice feature.