This one isn't super solid, but a rating from a classification board is a bit better than a rumor. Kind of.
The Australian Classification Board had a listing for Grand Theft Auto: Vice City pop up yesterday. In the listing it says that the game is a modified version. We have no idea what "modified" means for Rockstar's game, but it did get a MA 15+ rating, so we know it's not just a dumbed down GTA: Vice City.
This could be something juicy, like a PSN and XBLA release. Or it could be silly, like an iPhone title. We'll be watching out for this.