All you have to do is compare the games on each platform. The 3DS top 10 consists mostly of great exclusives like Monster Hunter 4, Kingdom Hearts, Resident Evil Revelations and Dragon Quest. Comparably the Vita has pretty poor exclusives like COD: Declassified (33 metacritic), Assassins Creed: Liberation (70)( and Ragnarok Odyssey(66) and then a bunch of late ports.
I think the Vita games sold good considering the majority of them shouldn't have sold at all. Also the fact that Persona 4 Golden outsold almost every other game in the franchise is a good testament to the systems ability to sell high quality 3rd party content. If only there were more of it (particularly in the form exclusives)







