Wrong. In both cases communist aggressors invaded a peaceful neighbor and we tried to help the locals resist being overrun.
South Vietnam was not a "peaceful neighbour" of the north, but an intregal part of the nation of Vietnam. It was artificially divided in order to allow the French less embarrassing exit in '54, just as Americans negoicated a less embarrassing exit in 1973. As for peaceful, I guess that is a matter of degree. The Vietnamese fought French colonialists, then Japanese invaders in 1940, then the French again after WW2, and then the Americans for another decade. They were no doubt getting a little hardened near the end of this process.
America did not intervene to support peace or freedom, but to fight communism. This was made very clear. Even today, Mr Flanders is repeating the idiotic nonsense that Vietnamese communists would be swimming ashore in California, if not stopped sooner (with a little rust on their AK-47s, no doubt). The South Vietnamese leadership was not democratic, and in fact the US has never made any bones about the fact that it would support those who supported US interests, not matter how brutal they were. I think it was FDR that issued the classic line: They may be bastards, but they are our bastards!
Korea, also, was a singular state, divided only for the purpose of disarming the Japanese still there at the end of WW2. It's true that the Soviets armed and supported their favoured man in the north, Kim il Sung, and eventually gave him the green light to invade the south. Stalin was at first apprehensive though, and wavered. But by 1950, the south was already in turmoil, as America could not accept the idea that Koreans were forming worker's collectives, and doing a pretty good job of running their own affairs. This sounded to much like "communism" to them, and so they disbanded them. To rub salt into the wounds, in some cases they re-appointed former Japanese occupyers to administrative positions, due to a lack of such personal on the ground at that time.
Would Stalin have given the go ahead if it were not already the case that some southerners had already taken up arms, and were shooting at Americans? We don't know, but we do know that once again, an astonishing lack of historical knowledge, and cultural sensitivity, and overriding self-interest, were factors in a war that might not have happened.
And yet in Both cases MILLIONS of Southern natives volunteered fought and died for their Southern Countries. Millions more in Vietnam were imprisoned after the South fell. More millions died risking life and limb to flee the supposed beneficial union between North and South.
"MILLIONS" of Vietnamese, in both the north and south, volunteered for the opposite cause. They fought on despite overwhelming odds- with nothing even approaching the air power, logistics, and heavy weapons available to the US, and its southern allies. But they endured. The south Vietnamese army, in contrast, tended to go downhill after US support was withdrawn. Clearly, some felt more strongly about their cause than others. Where they right? Not really. The point is that the US completely misunderstood and mishandled the entire episode, to the grief of many.
If you are now going to tell me that the US was in Vietnam for altruistic reasons, because here was injustice, and they were going to make it right, then please spare me the effort. Even the defense secretary at the time, George McNamara, later admitted that the US view of Indochina in the '60s, was ill informed, and the war was, in hindsight, a mistake. The US intervention was all about monolithic communism, a concept since discredited by historians and informed observers. It was foolish, it was mistaken.
If, in fact US foreign policy is all about altruism, then why Vietnam, or why Korea? There were, and there are today, areas of the world far more benighted than those particular countries, so why not put the resources were they will do the most good? You know the answer. It's not human need that is the priority, it is the perceived (rightly or wrongly) security and defense and trade requirements of the US that are front and center. If this was not the case, then Iraq and Afghanistan would not have happened, and Marines would now be in central Africa trying to stop the rape and murder there, or perhaps in Somalia, a road warrior movie brought to real life.