Contre Les Viets is a Vietnam variant of Arty Contliffe’s Crossfire. You need the original rules to play.
Incoming! is a set of modifications for using Arty Conliffe’s Crossfire game for the Vietnam War.
Games Workshop’s Lord of the Rings system proves to be remarkably versatile, as gamers have adapted it to a large number of skirmish periods.
The Vietnam variant of the Legends of the Old West game is available at the Lord of the Rings Battlegame Yahoo Group.
Fields of Fire is an online community for Vietnam wargamers. The site has a lively discussion forum, and focuses both on miniature wargames and on role playing games. Among the games supported are the classics Recon, from Palladium and RAFM’s Charlie Company, both of which I own and have enjoyed.
De Bellis Vietnam is a set of DBA-based quick and dirty rules for wargaming Vietnam.