It will be a full game. It uses a highly tweaked version of the GTA3 game engine. And also rockstar/take2 would be shooting themselves in the foot if they released it as a add-on, it would require anyone who wants to try vice city to have GTA3, which for the people that don't already own GTA3 will be a pain, most people wouldn't wanna buy GTA3 + the addon just to get vice city.
JayMan