The Viet Cong analogue is probably more relevant in Return of the Jedi, when the Ewoks fight the Stormtroopers. But even as regards the whole Original Trilogy, the “WW2 in space” analogy can only go so far (and refers mostly to the visuals and special effects), because the Original Trilogy is not about a war between political equals (despite the fact that A New Hope confusingly refers to a “Galactic Civil War” in the opening crawl). Rather, it is about asymmetric war between insurgents and an all-powerful totalitarian regime.
Yes the Ewoks, however poorly executed, at least resemble low-tech fighters against a much larger and more advance power. I just don’t see it as a metaphor for the US military action during that period. The iconography just doesn’t work.
However, the WW2 thing is more clear since you can take the Battle of Britain era with one small island against the whole of the Third Reich. Or resistance fighters in places like France and Norway. Later they gain more allies and fight on a more equal scale which turns the tide, etc. George saw movies about that in the 1950s after all.