Think you got mixed up a bit there.

Ken Burns Vietnam documentary series was about Vietnam (1960s - 1975 ?), not the Korean War (1950 - 1953), no?

The 320,000 South Korean troops who rotated through the Vietnam War over 10+ year period were great troops according to many US veterans who were there.

And also I do not agree with the broad generalization that South Vietnamese troops were hopeless.