Oh God, really?
The only people in the world who have ever managed to make a good 'Nam game were DICE, and Treyarch just barely managed to pull off a good World War 2 game. They really are so cocky from one good game that they think they're going to do what so many have failed at and make a good Vietnam game? Fat chance.
But I'll assume this is just a senseless rumor. And they had better not make some Cold War espionage game. That's so far from what a COD game should be I would have to firebomb Treyarch's headquarters.
Maybe, maybe there's a chance they could pull off a Korea game (since as far as I know no one has tried it before), but again, it's new territory and Treyarch needed to stand on Infinity Ward's shoulders to manage what tons of people have done before. Sounds to me like they're going to screw themselves over.