I assume they could work on HD universe maps like Los Santos and then use those assets in the final product so to speak with all the maps combined. Assuming they ever released Vice City in RAGE, can't see it honestly.
Because making sure to bring all the different cities of GTA in one game, without making one area too lifeless and copy paste streets over and over (ie. bring it to the quality level of GTAIV's city and V's - no regressions) is going to be a really big project. They will have Los Santos done with thanks to V, GTA3's Liberty City is also done by IV. All that might be left is Vice City and since they wanted a unified Los Angeles type deal for V instead of working with different cities we probably won't be needing San Fierro or Las Venturas.
By the time they get serious Xbox will no longer be living in a DVD world hopefully, and I hope they at least reconsider allowing mandatory installs. Cheap hdd's, why the fuck not?