One reason our moral legitimacy in Vietnam was undermined goes back to the early days. JFK, persuaded that it would please Vietnam's Buddhists, ordered the Catholic president of Vietnam, our ally, to be assassinated by the CIA. (Perhaps, too, he was worried that his own Catholicism was a political liability unless he bent over backward to be a bad Catholic.) That was a serious miscalculation, as well as a criminal act.
But at the time we pulled out, the South Vietnamese certainly were still willing to fight. The problem was in the U.S. Not that we "lacked moral legitimacy," but that the fellow travelers in the media persuaded many people (not a majority, even then) that we lacked moral legitimacy.
The whole picture of the war was enormously distorted by Hollywood, as well. Most of us get a kick out of watching "Apocalypse Now," but it's not exactly an accurate picture of allied military operations in Vietnam. The whole war was turned into a hippie caricature by clever propagandists, and the fallout of that deception is with us still. That's just one reason why John F'n Kerry can't be let off the hook.
I thank you both for making my point. Personally I think the moral legitimacy rested on helping South Vietnam resist the murderous tyranny of the communists in the North who were backed by other communists, Chinese and Soviet.
"I just love the smell of napalm in the morning."