CD Key BFBC 2 Vietnam

Hello! 

I bought a Battlefield Bad Company Vietnam code today. In game I pressed the button "Vietnam" and I toyed in my code after that I came in to the "Vietnam" login screen after that I pressed on the BFBC 2 button again (the one to the left) and now I can't go to Vietnam mode again because I need a new code says the client. I Tried to type the one I typed before again but I would not accept it. Now on the paper that the code is on it stands clearly that I can switch modes just by pressing the button!

Please help!