I know this has been brought up before but we did win vietnam, it just wasn't the whole knockout that America is used to (WWI and II). I think it would be interesting to tell it from this perspective I mean, imagine that your objective gets lost and you have to retreat in the face of overwhelming enemy numbers. I mean actually losing a few battles, not something that the CoD series does often. They could also add in a few new things, like combat fatigue because a big part of vietnam was the pychological warfare.