You cannot give Freedom to a people who have no desire to have Freedom for Themselves and Generations to come!
That can arguably apply to Afghanistan, but to a great extent not South Vietnam. Though its (US-installed) government was worthless, many South Vietnamese still had considerable nobility.
The ARVN fought for two years after the US left and the North invaded, before they basically ran out of supplies and had to surrender. And this was in the face of the North having virtually unlimited supply from the Soviet Union.
I will add as a note that the US Army was so incredibly bitter about the Democrat congress abandoning Vietnam and the men they had trained, that they went to extraordinary lengths to insure that the Iraq army would be set on a path of success ASAP. The US Army *assumed* that a Democrat congress would betray the Iraqis just as they had the Vietnamese, and wanted the Iraq army to prevail even if they were betrayed.