I dunno about that...between WWII and "Modern Shooters" there were quite a few Vietnam games...including Battlefield: Vietnam for that matter. Also, Vietcong comes to mind. There were a bunch of Vietnam mods for games like ArmA 2 as well.
Though I will say, I would welcome a WWII or Vietnam game with a modern engine like Frostbite 3.
The only Vietnam games i can remember are BFV, vietcong, shellshock, and a couple of mods. Which in my mind doesn't add up to quite a few, at least when you compare it to the plethora of WW2 and modern military shooters that have been released in the past decade.