Uh.. been a while since I played vice city but there should be a "saves" folder in C:\Documents and Settings\*your user name*\My Documents.
Or if you dont have admin rights it might be under the admins user name.
That is the location to the GTA San Andreas user files, and I think Vice City also puts them there. Inside the GTA SA, folder is the settings and saves... These examples are called "GTASAsf1.b" .... Range from 1 - 8 for the 8 save slots.
You can copy all thats in there or just the saves.. Put them on a disc and once you reinstall copy them back in the same folder and location...
*Note; You may need to start a new game and save before the folder is created by the newly installed game.