It probably won't go back to either of those as they are GTA lll era cities.
EDIT BY OTHER : Well, actually LC was one aswell, and both SA and VC are mentioned IN GTAIV, so actually there is a high possibility yes, but it may take place in a new city. But everyone wants another VC, so Rockstar is rumored to of began construction on the new GTA, but they are more concentrated on their new game "The Agent". So, there is a possibility of it being in VC OR SA, so keep tight and be prepared for a new GTA in about 1-4 years.
Good Night, and hope they do make another VC.