A mod for Grand Theft Auto V on PC allows you to hop in a plane in Los Santos and fly over to the Vice City map, which has been reproduced in GTA V, and there's even a trailer showing it happen.
You can read more about the mod here, and it aims to port both Liberty City and Vice City into Grand Theft Auto V. From the trailer, it looks as if it's coming along very nicely, although, as expected, there are a few bugs.
Which is the best location featured in a GTA game?