That's if the domino theory was ever correct in the first place. Yes, we did stop south korea from falling, but that's probably it. Vietnam, china and russia, while occasionally working together, were more than willing to fight each other. China and north korea were the only communist nations in asia that really worked together. The war between vietnam and china, and the border skirmishes between china and the soviets showed this. A nation becoming communist would have been more likely to occur due to the peoples desire and not any outside push. The domino theory isn't dependant on this, but if they had worked together more to spread communism it would have been more plausible.
The majority of the vietnamese people supported ho chi minh, though his support wasn't as strong in cities as it was in the country. The only reason there was a war was because diem, knowing he would lose, refused to hold elections that would have unified the country. That was why ho chi minh agreed at the geneva accords(against the wishes of many members of his army) to divide the country in the first place, because of the upcoming elections that would reunify the country. To the vietnamese it was a civil war, to them liberation could not happen as long as a colonial power (as we were seen as a continuation of foreign rule) had control. How was the war just considering we were fighting against the will of the people? It was not a war we, morally, should have won. This is different from what may have been best for vietnam, but if the democratic elections were held ho chi minh would have ruled all of vietnam, and not diem.