The question is nice, even if a little broad. It allows me to raise an important point about history: you might be taking consequences as causes.
You are asking: despite that US forces left Saigon, what are the long term and non-military factors that explained the fate of Korea and Vietnam? Surely, political and economical differences existed before their respectives 20th-century wars, and even more after. But the war was still a major step for both countries. And the outcome of the war was a major builder of the political and economical landscapes, in both countries, after 1953/1973.
The international situation during the Korea war was very different from the one during the Vietnam war. Along with pure military explanations (tactics, technology, guerilla...), this explained why in one place, the USA and their allies maintain the South governement, and in the other they left and thus failed to maintin it:
- First, the former ruler of Korea, Japan, was chased out by USSR and USA, and the country immediately divided. Then only, those two "half-country" fought together. In Vietnam, France was chased by local resistance (helped by China). It led to the division of the country, and the South part was backed up by the USA while the North part tried to invade it.
- Second, the involvement of world powers hidden on communist sides for both wars. For the "South" side, it was in Korea very open, not contested (except in France) and under UNO agreement. In Vietnam however, it was utterly contested, in USA as well as by other West or non-aligned countries.
- Third, while the fight in Korea occured in South and North territories, in Vietnam the USA were restrained for their actions on the North part. And when they let the reins go, they still limited to air and naval actions.
- Fourth point, while in Korea the ceasefire left place for huge tenseness, the limit line was somehow respected. And US troops stayed. In Vietnam however, the US wanted to leave, and find a ceasefire for a middle line just before living. After they left, the war started again and North Vietnam captured South: there was no American intervention (except for evacuation and in Cambodgia), no regular troops stayed in South Vietnam, and no political will wanted the US to guarantee the independence of South Korea by retaliation
So as you can see, the political situations were very different. And they are an important factor that causes the different fates of the country after the war, and thus all the political and economical differences we can see now.