I heard the same...and i really would like to see a "good" Vietnam one coz i canÂ´t see any WWII game anymore !
Nam is stupid
couldn't agree with Big more, either they stick to Modern Combat, as it seems the vast majority prefer that style game. Or go back to WWII and try and improve the game the best they can to make it as good as they can. Those two era's of warfare are the most popular, why would treyarch want to risk venturing out into new territory and risk making a massive failure, yes IW did it, but they obviously done research on what people wanted.
You never know what they're going to do until they confirm it themselves, this is most likely just a rumour.
HOWEVER - it is also likely that Vietnam is the setting for the next COD game BUT remember the biggest success of WAW - Nazi Zombies. They're obviously going to bring that back into their new title somehow, why would they abandon such an opportunity to make more money? Plus the fanbase for zombies was huge.