Vietnam was lost because of Communists Dems "Operation Purse Strings" got elected and cut off support to the then victorous S. Vietnames. The war was won, the Paris Accords were signed, game over.
Paris Accords were signed, game over.
The game was over long before the Paris Accords. As I said, Johnson would not allow the US to win. Handcuffed the military. Remember the NVA was shooting at the last planes leaving Saigon.