South Korea is expected to have roughly a $35,000 GDP per capita this year (barely behind Japan at $39,000; they're certain to overtake Japan this decade).
Vietnam is at around $4,100.
It's definitely implausible that South Korea would be anywhere near so well off had the US and UN forces not prevented North Korea from conquering them.
One wonders, regardless of regime, how much better off North Korea would be if it similar trade opportunities as South Korea. Or how much worse South Korea might be without generous western economic aid.
UAE/Qatar/Saudi/Kuwait for example, are doing well on a per-capita GDP basis despite being absolute monarchies / authoritarian states. Perhaps their ruling families are a bit better than the Kim family, but perhaps the Kim family would have been better if they’d received similar levels of western support and constant access to top advisors instead of isolation.
I don't get that part of your (otherwise excellent) comment. If the US (and the UN) hadn't taken part in the war in Korea, South Korea wouldn't exist and the reunified country would have looked like something like either current-day North Korea or China and might not have gone through a similar economic boom, which allowed the intellectual capacity to get to work (and export the results of their work, which wouldn't have happened in a North Korea-style outcome)