I think I can answer that one.
Saved games will remain as they were, unaffected, until/unless you want to use them.
Games that are IN Monte Vista, simply won't be able to be opened at all.
(until you reinstall Monte Vista-- then they'll open fine. they won't be touched otherwise)
Games that are not in Monte Vista, but have items from the Monte Vista expansion, if you should open them, will lose those items. The game will try to seek out generic replacements, and will kindly inform you that it has done this. If you don't save over those games, though, and reopen after reinstalling Monte Vista-- they'll keep those items.
The game will not change any of your saves unless you open them, and then resave.
I know because I had a saved game with Ambitions and Late Night installed, then opened it on another computer without Ambitions or Late Night. Saved games that were in the particular towns could not be opened; saved games in say, Riverview, would open, but any Late Night or Ambitions content got replaced, and any Late Night or Ambitions skills were lost.
If you're worried, just move a copy of your saves to another folder.