Steam – “Vietnam” from Steam with retail base game

It depends on what game it is. Obviously Valve's own games are possible to register on Steam. Call of Duty: Modern Warfare 2 also comes to mind since that one has to be played via Steam.

Here is a list of all retail games that can be activated on Steam: https://support.steampowered.com/kb_article.php?ref=7480-wusf-3601

Well, it's a remake of 2004's Battlefield Vietnam only in the broad sense that it's a similar setting.

I do not think it's a true remake -- in the sense of having the same maps, same gameplay, and exactly the same vehicles and weapons as the 2004 game.

From what I have read, it'll be the same (awesome) core BF:BC2 gameplay, but transplanted to new jungle maps, vietnam-era weapons, and vietnam-era vehicles.

The [Vietnam] expansion pack will contain 4 new maps, 6 new vehicles, and 15 new weapons. It is also planned to include 49 contemporary music tracks which can be listened to via an in-game radio as well as new voice overs for both American and Vietnamese forces, based on authentic communication from the Vietnam war.