However, the U.S. was living in the shadow of the so-called domino theory. And that is why they thought they had to act.
That is close, but that is not quite correct.
In the aftermath of WWII, quite often when a nation broke away from Colonialism, it picked a sponsor to assist it until they were on their feet. And after WWII, more often than not the nations elected either the Soviet Union or US to assist them in that transition.
Both North Korea and North Vietnam picked the USSR after breaking away from Japan and France. Both South Korea and South Vietnam chose the US as their sponsor. Laos also decided to work with the US, but not make a formal alliance. And a lot of people forget that North Vietnam invaded them also.
Cambodia tried to steer a neutral path, and did not join either the US, USSR, or any of their partners with alliances. Which is why then they fell in 1975 to the Khmer Rouge, the world pretty much did not care.
With Vietnam, it is more like in Korea. An ally of the US was attacked, so we responded with assistance. Laos was less of an ally, so we gave them less assistance. Cambodia was an ally to nobody, so nobody gave them assistance.
This is something many who try to point at the "Domino Theory" tend to miss. Because if it was as they believe, then the US should have been invading Cambodia when it became Kampuchea. Of course, it did not matter much that the Soviets did not care for Kampuchea either, nor did the North Vietnamese. Because even though they were "Communists", they were the wrong form of Communists, and closer to the PRC than the USSR.
And this can even be seen in Ukraine today. For over 3 decades, they (as well as nations like Sweden and Finland) tried to steer a neutral course, and not aligning with either USSR-Russia or US-NATO. And in the end, that is why many nations are offering to support them with equipment, but not a single soldier. Something I actually agree with, because one can not expect others to be willing to die for their nation if they are not willing to do the same in return. I would have had no problem fighting to defend one of our many allies, because I know they have agreed to do the same in return. For others, they are more than welcome to any equipment we are not using, but as far as I am concerned that is where it ends.