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The Battlefield: Bad Company 2 Vietnam Expansion DLC is on sale at Direct2Drive for their 30 Days of Deals (Deal$) sale for $7.48 ($14.95). Use the 15% off coupon 7712FAABF90A44AAAA40773EE6B60AE5 to make it $6.36.
The expansion requires vanilla Bad Company 2 which is on sale at Amazon currently if you need to pick that up. Check the Promos and Coupon codes page for the link to generate a new 15% off code if the above is expired.
The game is also on sale at IGN PC Store for $7.48 but there’s no coupons for that site that you can use unless you can get your total over $19.99.