You kidding? The Vietnamese communists killed half a million people then killed another half-million in Cambodia. Some murdered some worked to death in slave camps.
Setting aside for now the question of who actually is responsible for those deaths, or even analyzing the numbers, lets just take a look at
when.
We started sending military advisers to help the blameless French against those dirty commies as far back as 1950. How many Vietnamese had been killed by the commies by then? How many had been killed by the French and their Vietnamese collaborators?
How many Cambodians had been killed by the commies?
We started massively increasing the number of our "advisers" in 1960. How many Vietnamese had been killed by the commies by then? How many by the remains of the French occupation government?
How many Cambodians had been killed by the commies?
If the head count was our justification for entering into the war then these questions are important, at least by your logic. If the deaths you seem so concerned about had not happened
yet, or did not result solely at the hands of the communists then your objection lacks substance.
How many Vietnamese would
not have been killed if we had not taken part in that conflict? How many were killed by us or because of our interference?
How many Cambodians?
Apologists for our misguided adventurism in Vietnam find any discussion of the history of the region to be inconvenient. This is because the root cause of the conflict was the century of brutal conquest and occupation of the country by the French, and the vigorous resistance to that occupation. When the Vichy French occupiers capitulated to the Japanese during WWII without so much as a whimper, and eagerly collaborated with them for the duration of that war it was the Vietnamese resistance which we were allied with. Not merely an alliance of convenience, but one of ideological alignment as well, at least insofar as their independence was concerned. It was at this stage that we led them to believe we would support their efforts toward freedom from French domination. These assurances were made in full knowledge of their sociopolitical philosophies. This later became inconvenient for reasons which were largely unrelated to those philosophies in anything but name.
Almost on the heels of WWII the French set out on a purely military campaign to try and regain their lost colonial holdings. They didn't fare much better than we did less than two decades later, in spite of the fact that their effort was largely funded by U.S. dollars. The objections of the French public to the war were eerily similar to our own in a later decade. With good reason.
It is important to note that it was the remnants of those same collaborators that we were propping up in the south after the French were so humiliatingly defeated by the nationalists.
If death tolls are the gold standard by which you judge the merit of our interference in foreign disputes then you need to begin by considering the deaths of Vietnamese during a century of French repression, and the deaths incurred during the failed attempt by the French to re-occupy a hostile country.
Then start looking into who is really responsible for the deaths you chose to cite.