There were two distinct periods as far as as comic books went, pre- and post-1968. In 1968, beginning with the Tet Offensive, the U.S. dramatically increased its involvement in Vietnam, calling up many more troops than had been committed before that. Prior to that, to most Americans, the war in Vietnam was a distant thing, not uppermost in their consciousness. During that time, the older generation of comics pros with an anti-Communist stance held over from the 1950s would flirt with Vietnam stories, treating them no differently than other stories about other wars. Marvel even sent Iron Man, Captain America, and Thor to Vietnam in various one-off stories. There can be no doubt about Stan Lee's arch anti-Communist approach in the early 1960s Iron Man and Hulk stories, featuring as they did Tony Stark, a military industrialist with government contracts, and Bruce Banner, a civilian scientist working on U.S. military projects, although Vietnam only featured in a few stories, with the bulk of them concentrated on the threat from Russia. Chinese Communism seemed more of a nebulous danger, but then again, there was The Mandarin in Iron Man (yet another variation on Fu Manchu, as was the Yellow Claw before him). DC published a war series about Captain Hunter's Hellcats, but it didn't go over well.
After 1968 when things started getting serious, Americans as a whole became polarized over the issue of the U.S.'s involvement in Vietnam, coining the phrase "Hawks and Doves", with conservatives sticking to "My country, do or die" and protecting the world from the global threat of Communism, while a large number of people questioned the morality of American citizens fighting and dying so far from home in an undeclared war in which we had never been attacked. By that point, most comic book publishers had decided to stay out of it.