Only 10% of South Vietnamese supported the communists [half the total in France in that period!]
A terror group - the Viet Minh [later Vietcong] worked inside the nation to bring it down so that it could be attached to the already communist North run by the dreadful Ho Chi Minh. The will to follow through in that war simply did not exist - and it was lost!
Most afghanis do not support Al Qaeda!
Was the Vietnam War morally correct? - Arguably so. Was it worth 55,000 American dead - surely not!
The war could have been compared to the earlier war in Korea where the UN had stepped in to stop the predations of China and N. Korea in the early 50s - but it was 'too tired' to do the same in Vietnam - where there was good cause to have faith in the much sneered at 'Domino Theory'.
Similarly, Afghanistan is an international concern where, by and large, only the Yanks and Brits are taking it seriously.
Is the war morally justifiable? - I think so. Just about. But where are the other nations?
"We've done our bit - now it's time to quit!"