Thanks for the plug, Hoagie!
We are indeed playing a Vietnam campaign using Nam '68 Tour of Duty. Our 5th monthly game is set for this coming Friday. See my thread for details...
As for rules, there are several serviceable sets out there. Way back when, I did 20mm Vietnam games using Charlie Company and Force on Force. More recently, in 28mm, we've used The Nam (by Wayne Bollands), Fistful of Lead: Bigger Battles, Bohica, and, now Nam '68. All have their strengths and were fun to play. And, all can be played in 15mm, 20mm, or 28mm, pretty much straight out of the proverbial box...
I've not played Flames of War's Vietnam game, though (although I do own it).