What I think we'll see from GTA on the PS Vita
The game will be set in a modern day Vice City so that the HD era comes full circle (GTA IV was set in Liberty City and GTA V in the Southern California equivalent of San Andreas). It will be bigger than the previous iteration of Vice City and maybe even feature swamps.
We probably won't see multiple protagonists but if we do, it could be a Miami Vice thing with the story focusing on two protagonists instead of three. Focusing on one protagonist again wouldn't be a bad thing though.
Visually, I don't think the game will look as good as GTA V but with Vice City's vibrant neon lights, warm weather and populous beaches it will still feel like a new game. I think it'll really compliment the PS Vita's OLED screen.
I'm not really sure what to expect music-wise.