Honestly, the reason for the Vita's being outsold is obvious and twofold:
1. It has few 'good' exclusives. Or at least, ones that people are willing to buy a console for.
2. The games on the console are the wrong kind of games. What do I mean by this? I mean that Sony's screwing up by releasing what are either ports of home console games or games that would work better on a home console, whereas most handheld console buyers want games specifically designed for a handheld. You know, pick up and play, easy to save and restart playing, not requiring a lot of concentration.
That and it doesn't have Nintendo's franchises on it, nearly any console with a Pokemon or Mario game is basically unlikely to fail.