Ugh if you originally bought it on origins or ea downloader in the past, the key will only be shown as used. So basically it only works for any key obtained outside of origins.
That sucks. I bought it on EA Downloader. I logged into EA Downloader today and it prompted me to update then it showed my BC2 game linked to Origin along with the activation key. I tried entering that into origin again and it said it was already used. I guess I can't get Vietnam then for free.
EDIT: Oh I see this only works if you bought Vietnam before. Therefore you do not get Vietnam for free, title is misleading