Monster Hunter would make Vita explode in Japan.
An exclusive GTA would do great things also. San Andreas stories? I would love that.
Uncharted definitely sold systems, another Uncharted on Vita would give it a significant boost.
I don't think the Vita has many obvious system sellers, it's the library as a whole that will sell it. I think very few games will do much on there own, some people may see Bioshock (for example) and want it for that particular game, but I think what will happen far more often is people will see Bioshock, Uncharted, GTA, AC, Motorstorm RC, Resistance, Gravity Daze, Unit 13 LBP, Call of Duty, Escape Plan etc. + E3 announcements... all these great games on the system, it may not be one particular game that pulls people, but the library as a whole. Once these games have been seen in action, some have been released and Vita gets a price drop, it'll do very well.