Tanooki

Hell, make Vice City a separate, online version of GTA 6's map from the start, like they eventually did with GTA 5. Have it use the same data assets and engine as the main game, and drop a huge update for GTA 5 online, updating it to the new game engine, but allow players to choose whether to use the old interface and features, or the new set.

Then, price the Vice City expansion the same as Online currently is, and call it the "Airline Pass" or something, having it available as an addon to either 6 or Online, or as a standalone, which could later link to 5's Online. Then, in-game, to switch between the two, you go to the airport, buy a ticket (Maybe 5 GTA dollars or something cheap), and fly to the other city, with a short (skippable) cutscene. Optionally, switching could be available from the menu, but where's the fun in that?