But the similarities aren't just cosmetic; they are also all over the plot. Like in Vietnam, the technologically superior soldiers soon find themselves overtaken by an enemy that tends to sneak up on them in the dark. Obviously, Cameron isn't saying that the Vietnamese were penis-headed rape monsters -- it's more about the attitude of the soldiers towards them, which goes from "I'm the ultimate badass!" to "Game over, man!" over the course of the conflict.
Then there's the way the movie portrays figures of authority: They're all corrupt, useless morons, basically. The Weyland-Yutani Corporation, like many major companies during Vietnam, is putting their soldiers in jeopardy just to make a profit. The commanding officer, Lieutenant Gorman, is not only elitist (he chooses not to eat with his men, which pisses off Hicks), but laughably incompetent: He gets himself knocked out during the very first fight, and Ripley has to rescue everyone. This is most likely based on what most people think of American officers during Vietnam: that they got a bunch of young Americans killed without ever actually getting their hands dirty.
20th Century Fox
The scene where Paul Reiser does an impression of Nixon for 10 minutes should have tipped us off.
It's probably safe to say that Cameron doesn't think Vietnam ended too well, considering how the colony on LV-426 is engulfed in a thermonuclear explosion at the end. And those poor marines didn't even get to do any surfing.