You might be able to find a used copy from Amazon or Ebay or something similar.
I had to use a mod fix to be able to play it on Win 7 64bit... don't have that link anymore,
but will try to find.
It is such a great, well designed game, good storyline and action. Although the expansion was
not very good, compared to the main campaign. it did give you some added weapons and maps
to play in the skirmish modes of the game. The coop is (was) a lot of fun too, back in the day.
There is no other game set in the Vietnam war that comes close to the experience created
in Vietcong. Even Vietcong 2 didn't meet the expectations or the quality of the first game's campaign.
Edit... As far as it coming to GoG, that all depends on who owns the rights to it.
You can vote for it here... http://www.gog.com/wishlist/games/vietcong