I'm going to take a different stand here, and argue that our involvement in Vietnam was the action of an empire, protecting its resources and trade routes rather than responding to a true security threat to the United States. Every empire acts in such a way, as it is the nature of empires, but I do not support such action currently, nor do I think it was justified then, especially, in hindsight, given the human, financial and environmental costs of such a war.
At the time, context and contingency becomes critical. The fear of Communism and the belief in the domino effect were both perceived as real and valid threats. In this light, protection and insulation against these threats were seen as justified. It is really difficult to be able to look past such contingencies when we are placed within the midst of these situations. In such a light, the case can be made that involvement in the war was justified. We now know better on many levels. We understand that the fight for the Vietnamese was not about Communism, but rather for independence. We also understand that Communism ended up fading away, but again, this is something that could not have been known at the time.
If you look at it given what we know now, then of course it was not. We can see this because the domino effect never really happened. We lost South Vietnam to the communists but really nothing bad happened internationally. Our allies in Asia were never really threatened. Today, we do a lot of trading with communist Vietnam and it's no big deal. Today we know that the Russians and the Chinese were'nt really together in a plot to dominate the world and we know that the Vietnamese did not just let those two big countries control them.
But it you look at it from the perspective of the time in which the decision to get involved was made, it's a lot harder to say that we should not have gotten involved. The domino theory made a lot of sense if you just thought about it. Smart people believed it was true. Sure, some people didn't, but it would have been a heck of a gamble to just discount the idea of the domino theory and potentially risk letting the Soviet Union dominate all of Asia.
Seen in retrospect, it is clear that Vietnam war did not benefit anyone - USA, Vietnam, or the the communists, whom USA was opposing. Thus it is very clear that Vietnam war was totally futile. As a matter of fact it appears to me that the various proxy wars between supporters and opponents of communism fought in countries like Korea and Afghanistan have all been miserable failure. All these wars, fought as a part of the cold war were directed at escalating the tension that existed between different parts of the countries. Further, these wars were fought half heartedly by outside opponents as well as supporters of communism. As a result none of these wars have helped anyone.