Navigate to C:\Program Files\Rockstar Games\GTA Vice City - Definitive Edition\ and look for the Paks folder. Most mods drop in a ~mods folder here.

No. Unfortunately, Rockstar does not sell the games individually. You must buy the whole trilogy. However, you can launch Vice City specifically from the launcher without installing the other two.

Before you click a single download link, you need to understand what you are getting. The Definitive Edition is just a mod pack. It is a ground-up remaster using Unreal Engine 4.