Only 1 of the top 4 selling Vita games is made by a company other than Sony, Fifa Soccer. Furthermore, only three other 3rd party games fill the next six spots. So in total, only four 3rd party Vita games have sold over 200,000 copies.
Contrast that to the 3DS. Even if you double the 200,00 copies to 400,000 copies to compensate for the 3DS's being out for twice as long, you still have a dozen 3rd party games with that many sales: Monster Hunter 3G, Kingdom Hearts 3D, Dragon Quest Monsters 3D, Super Street Fighter 4 3D, Mario & Sonic at the London 2012 Olympic Games, Lego Star Wars 3, RE: Revelations, Professor Layton 5, Sonic Generations, Rayman 3D, Inazuma Eleven Go, Ridge Racer 3D, and Super Monkey Ball 3D.
So my question is, why do 3rd parties even bother with Sony systems? It's obvious that if they want to sell games, they have to do it on Nintendo platforms.