What Andre said pretty much. I don't think Sony consoles rely as much on 'killer apps' as much as Nintendo or Microsoft do.
For example, when Halo 3 came out it pushed 360 sales through the roof. I'd imagine Pokemon 3DS would do the same if it ever comes out for that console.
There isn't really the same kind of thing for Sony consoles. Gran Turismo maybe? Uncharted perhaps? Not so much though.
Getting a GTA game for the west and Monster Hunter for Japan would be a good start. I think it'll be more a case of the Vita building up a library of exclusives, so that someone thinking about buying it will say "Actually, it has LittleBigPlanet, Gravity Rush, WipEout, Uncharted, Fifa and GTA now. That warrants a purchase."
It'll be a slow burner rather than a flash-buying phenomenon, and that's why I'm trying to avoid all the "VITA IS DYING" stuff I see all over here and reddit because it's far too premature at this point.