Zombies have multiple roles in Vice City's Serbian. Whether it is a small little radio joke, or an actual threat, is determined on what you are doing. Zombies are NOT actually part of the story, and are NOT in single player.
Some radio show hosts talk about zombies in two forms. One talks about zombies survivng on a cruise ship lost in the bermuda triangle. While the other hosts are talking about the upcoming movie "AidZ part 3: Condoms don't protect bites".
Players can play as a zombie in multiplayer if they desire. So, they can select the zombie model and destroy the city with their human friends. Being a zombie does not change the gameplay in any form.