Vice City originally started off as a GTA 3 mission pack

destructoid.com:

We’re not far off having our eyeballs exposed to the first official GTA 6 trailer. Until then, let’s talk about the classic games in Rockstar’s iconic series. Hey, did you know there was a point when Vice City (easily the best entry – I will fight you all) nearly wasn’t its own game?

According to a blog post from Obbe Vermeij – former technical director at Rockstar Games – the follow-up to Grand Theft Auto 3 was just going to be a mission pack. Vermeij said the team had the idea to have a game set in the 80s in Miami. But it wasn’t until GTA 3 became as successful as it did that a decision was made to turn Vice City into a standalone entry.

Just started a blog about my time at Rockstar North.https://t.co/F23oAx17tr#GTA

The original project was already six months into development, but artists and level designers were “able to get straight onto” the next in-line for the GTA franchise. Despite it being built off the third game’s engine, Vermeij remembers VC being “more stressful” than Grand Theft Auto 3.

In an earlier blog, we learn that the studio was keen to try something non-GTA. Inspired by the undead, Rockstar wanted to run with the idea of creating a zombie game that would have been built off the Vice City code.

Vermeij describes the elusive project called “Z” (pronounced either “Zed” or “Zee”) thus:

The game was to take place on a windswept foggy Scottish island. The player would be under constant attack from zombies. The player would need to use vehicles to get around but vehicles would need fuel. Acquiring the fuel would be a big part of the game.

Sadly, Z was abandoned a month into development as it was “depressing,” and the team got to work on San Andreas. Swings and roundabouts, I suppose.