"Vietnam" from Steam with retail base game?

Steam had EA Week, they had all of Battlefield stuff on sale, including "Vietnam" expansion pack for Bad Company 2 at 50% off.

Is it possible to use Steam version of that expansion pack with retail version of BF:BC2? I know that this "expansion" is basically just a key which you register in game, but how does that work in case of Steam version?

EDIT: the sale is over now.

I have come across this issue before when I had the Dragon Age on Disk but wanted to use the expansion on Steam. In my case it was not possible and I needed the Steam version of Dragon Age in order to use the expansion. Steam has been putting notes in the details of the game to make things more clear about it. You can see a notice in steam under the game media that states the following: "Notice: Vietnam requires the base game Battlefield: Bad Company 2 ON Steam to play"

You can view this link to the Steam page for the Battlefield expansion to see the Notice. http://store.steampowered.com/app/47880/?snr=1_4_4__13

Short Answer: Unfortunately you can't use the Steam expansion with that Disk.