If i have BFBC2 on CD and i buy BFBC2 Vietnam expansion pack on steam will it work with my BFBC2 copy?

BFBC2 Vietnam is actually already on your machine (assuming you update). the map pack 7 also contained all of the Vietnam content, the only thing the expansion pack actually does is unlock it for your enjoyment. however i play on ps3 so im not 100% about pc but i would assume yes. (i don't think you can even buy a disk with the expansion)