Use that exact link to get the promo price of £4.99. Then enter this code:
It doesn't matter where you get your vietnam code, as long as you have BC2, you enter your code ingame. So if you have retail or steam version this code works with no problems.
Might worth trying to get TCB as well: http://www.topcashback.co.uk/ea_store_uk/
p.s. I'm kicking myself because I just bought it for £7.49 from the store with the code, and then after a 2 hour session came back to the store and saw this promo offer.
***Updated to reflect new pricing***