I am not sure that one massacre, no matter how bad it is, defines a whole war. The Vietnam war was meant to block the expansion of communism which at that time had a fair amount of blood on its hand.
I'm not sure the blocking of communism is as noble as a cause it was proclaimed to be.
Most of the anti-communist fear in the US was pointless, and arbitrary. Communism only served as a threat to the US in so as much as it was on paper not what the US adopted, and therefore the US had to be against it to maintain its own legitimacy. But in the end there's not much difference between the US and China (a so called communist country) except that China is less developed and the market is more tightly controlled than the US market. But as you can see China has trended to the mix of market freedom and control similar to that of the US and the world didn't end.
In practice, communist countries were more authoritarian military dictatorships more than anything and if the country did bad things it was more or less due to the regime rather than communism. And the US would have been fine with such regimes at the time so long as it wasn't communist. For example, South Korea was a dictatorship until the 90s.
