The south wasn't as outright ruthless as the North was, but that doesn't mean it was a happy place of freedom and democracy. Again, the military junta won the election, and it did imprison its primary opposition. Just because they weren't taken out and immediately shot (mostly because the US wouldn't countenance it), doesn't mean it was a functioning democracy.
Before that, South Vietnam was ruled via a series of military leaders who gained power through coups and before that was run by Diem who basically created South Vietnam by unilateral decree and ran a massively fixed election earlier.
As for South Vietnam failing because it was a fledgling democracy poor at running a war machine, that's very wishful thinking. The government was run by the military, had been for years. A military general was in power with degrees of control over the state and media far in excess of what any democracy could be legitimately said to have. They had huge amounts of equipment and huge numbers of troops.
Their problem was a corrupt officer corps and a government run by said corrupt officer corps. There's a reason Thiệu was not popular in Vietnamese expat communities in the US.
As for Korea, no, I haven't said anyone was legitimate in anything, I've freely admitted that both North and South in Vietnam were gigantic assholes. However, there are huge differences between Minh and Sung. Sung was a puppet leader from the very beginning, despite much revisionist history on their part, Kim Il Sung had basically zero experience in anything aside from working for the Soviet Union before being installed as puppet leader of North Korea, and had Soviet pilots fighting for his state and millions of Chinese troops come to his rescue. Minh had tons of combat experience and lots of political engagement abroad, he was supported by the Soviets and Chinese (and, funnily enough, the US OSS during WW2) rather than installed, and he received nowhere near the material or economic support that North Korea or South Vietnam did. North
Korea and South Korea were both hugely terrible to their populations, in the 1950's it's hard to say who was worse, and until the 1970's the North recovered better, though now it's difficult to even consider North Korea communist, they don't even make a pretense of following Marx or Lenin anymore (they've even removed their portraits from public places), at this point they're basically a Divine Right Absolute Monarchy.
Ultimately, in North Korea, it was a situation of the Soviets and Chinese going "we want X to happen, put Y guy in charge to make it happen, give him Z marching orders", whereas in North Vietnam it was "hey, guy A is doing stuff we like, we like where this is going, lets give him B support and that'll honk off our nemesis C".
That said, I'm not also saying that the North in Vietnam was justified in attacking the south, but they did have their own objectives that they pursued that were not set for them in Moscow or Beijing. I haven't stated anywhere that the South was justified in being conquered by the North.
Really, the bigger point is that both acted like assholes, one was fighting for Unity (which is different than saying Unity should happen under their terms) and to remove foreign troops, and another was fighting for its own political power and relying on said foreign troops to survive.