Vietnam?
Us getting our asses handed to us?
Yeah, I don't know if making a Vietnam game was the best choice. I mean, I'm happy it's not a WWII game, but Vietnam might be a tricky road to navigate for a couple of reasons.
'Nam is still a sore-spot for many Americans, especially so in this politically-charged day and age. We were never technically at war with Vietnam, our reasons for being there were suspect at best, and the dislike of that war in southeastern Asia is clearly still a source of discomfort for many, even reaching as far as people calling the Iraqi war "this generation's Vietnam".
So how they tackle the storyline will be interesting, if not tricky. In WWII games, it's easy: almost everyone in America believes or has been taught that entering the war was just, and the enemy was easy to hate (the Holocaust, Pearl Harbor, lebensraum, etc). In the MW series, since the story is entirely fictional, the enemies can be made to be clear-cut villains in the strictest sense of the word, causing little to no amount of second-guessing on the part of the players during the story.
With 'Nam, it's an entirely different story. The political aspect of the war is cloudy at best, and the war itself was particularly brutal on US troops, another source of disdain for the engagement then and even now.
This is to say nothing of how the actual gameplay will be altered by setting a war in Vietnam. During the war, the enemy was well hidden both in terms of the jungle setting and the inability to differentiate between non-combatants and the enemy. Battles were bloody and slow-moving, and the tactics used by both the Vietcong and US troops were sometimes questionable at best.
This could either be a game that everyone comes to respect for its "realism", or one people dislike if only for the topic it covers.