The rumor sounds plausible. GTAV already had multiple playable characters, so if they went that route again the likelihood of a female character being in the mix rises significantly. Two distinct locations have already been done with Los Santos and North Yankton, too, the difference being that they would need to pour a lot more time into fleshing that second area out and that's why the '21-'22 launch makes sense, not to mention the insane amount of money GTA Online is still bringing in.
Can't say I'm placing any real weight behind this rumor, but I would love to see Vice City get the full HD treatment, just so long as they include taxi, police, ambulance and firefighter mini games/side missions as well as the 80's setting.