David Horowitz has an interesting view on the American Left and domestic opposition to the Vietnam War:
In the Vietnam War the United States was supporting a dictatorship in South Vietnam on the grounds that the dictatorship was anti-Communist. “New Leftists” who believed by and large that Communism was a flawed attempt to create societies governed by the principles of equality and justice had an argument (whether one considers it plausible or not) for opposing the United States defense of the South Vietnamese regime. Perhaps (so they reasoned) a victory for the guerrilla forces of the National Liberation Front of South Vietnam would mean the emergence of a society that honored the principles of equality and justice. This was an incentive to see that America was defeated. And this indeed is the delusional vision that motivated people like Tom Hayden and Jane Fonda other anti-war activists.
First off, Edward Herman wrote in Demonstration Elections about the US-staged elections in Vietnam:
Staged elections are not new. They are “demonstration elections” and have been around for a long period of time. From Vietnam in the 1960s to the recent Afghanistan elections. “The purpose of these elections - crafted by the US - was to persuade US citizens and especially Congress that we were invading these countries and supporting a savage war against government opponents at the invitation of a legitimate, freely elected government. The main purpose of a demonstration election is to legitimize an invasion and occupation, not to choose a new government”, wrote Edward Herman and Frank Brodhead.
For the Americans, “what happens before or after the elections concerns them not one iota. What matters is that the elections become a good PR exercise for the Bush administration”, Wamid Nadhim of Baghdad University told Al-Ahram Weekly. Furthermore, these elections are also seen as an excuse for Tony Blair and other “coalition of the willing” leaders to justify their support for Washington’s illegal war of aggression and occupation.
So yes, Horowitz' statement is literally correct, the US was supporting a dictatorship in Vietnam, but as Edward Herman and Frank Brodhead point out, the US leadership expended a good deal of time and effort to convince the American people that they were supporting a democracy there.
The idea that leftists among the American opposition to the war were trying to defend Vietnam's independence because Vietnam was trying to defend “the principles of equality and justice” is an interesting notion. Personally, I was only 15 when Saigon fell, but I've read a great deal about the Vietnam War since then and I'm still under the impression that I developed at a young age when I concluded that Americans were simply not wanted in Vietnam. The American soldiers were not welcome there, even though American soldiers, as individual persons, demonstrated all of the fine qualities that US myth and legend have talked about for the past two-plus centuries.
I also read the Port Huron Statement a few years back, the philosophical basis for the leftism of the 60s and I don't remember much there about anyone defending Vietnam because it was such a marvelous society. I remember the radicals of the 60s being very concerned about American society and how it could be improved.
Seems to me that the Left wanted the US out of Vietnam as 1. American troops were not wanted there and 2. Because America was expending a huge amount of blood and treasure there for no discernible purpose. As Steve Gilliard points out, there are also many other purely military problems with Vietnam then that are hurting us in Iraq now.