Theres two stories going around at the moment...
1: Vice City is in the works for the PC and will be released 6 months after the PS2 version (Around April next year). Same situation as GTA 3. That Sony 'Exclusive' deal is only supposed to be for consoles.
2: That Sony Exclusive deal is a blanket agreement between Sony & Take 2 meaning no Vice City for anyone other then PS2 owners