Now that we've got the figurative meaning of "Fantasy Viet Nam" out of the way, I think the idea of a campaign set in a parallel fantasy universe, where the PCs are in a Viet Nam like country, drafted as special forces or whatever, to fight the inhuman hordes.
Toss in a tunnel network so that the "monsters" can pop up most anywhere, abusive "charm person" abilities so that traitors and informants are common, desperate men-at-arms engaging in every more bizarre/draconian behavior to "win" the unwinnable war...just seems like there's a real campaign there.
Or a nightmare. Whatever.