I mean what else is on the Vita that's so much better? FFX and Persona 4? Sure, but I killed those games already with multiple playthroughs on other systems already. Yes it's cool they are portable, but I'd rather spend time playing new games.
Ys? Yeah I bought a Vita for it and it was pretty solid.
Tales of Hearts? Sure. Got it. It's a Tales game, which means it's a pretty run of the mill bland game.
Demon Gaze? Yeah no. Dungeon crawlers are boring to me and I doubt the subject material is going to mix better then Neptunnia.
Conception II: Children of the Seven Stars? How about no.
Mind Zero? Yeah, low rent Persona dosen't appeal to me.
I mean what's so much better then Neptunnia that's on the system. All of it's jrpgs hover around that c-tier area and/or are filled with fan service. I'm not sure how Neptunnia is bottom of the barrel compared to the rest of the system. Pretty much all of the jrpg library on the system is low budget super niche games.
I'm not saying it has a strong jrpg library, because it doesn't, at least not in terms of localized games. I'm just saying Neptunia is ass. I also primarily bought my vita for Ys Celceta.