Is Vietnam addon for Bad Company 2 will be a remake of an original Battlefield: Vietnam game (similar to how BF 1943 was to BF 1942), or it's a completely different game? If it's a remake then what features will be carried over?
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.