South Korea is an economic powerhouse, while North Korea is a ******** where everybody is starving to death. If you remember, South Korea had some unsavory characters running it as well. However, because the US stuck with them, they have a much more stable government.
South Vietnam had some lousy leaders as well. However, they were Pro-American and we would've influenced their government the same way we influenced the South Koreans. As it is, we made a promise to the Vietnamese and Cambodians, and we reneged on that promise. To this day, there are many countries that are still wary of getting into an alliance with us.