Grand Theft Auto 5 is not only going to include San Andreas but also some ski resort areas that represent ski resorts in California, and some areas of San Francisco will be included. There are going to be flyable airplanes and a lot of new vehicles, like jet skis and a ski chairlift .
To the poster who asked :How would they incorporate the GTA Vice City world into a newer game?", they could just use modern Miami Beach. There's still just as many drug dealers here as there were in the 80's and Ocean Drive still basically looks the same now as it did in Vice City.