The South Vietnamese fought almost two years after we left. It was the pulling of our military assistance from the ARVN and the failure to do anything about the North Vietnamese violation of the peace agreement that led to the downfall of South Vietnam.
Here is one of the best references on the chronology of the Vietnam War.
I spent a year in-country 1967-68 and nine months off the coast on an LPH. I refuse to watch anything that Burns produces. He is not a real historian, but rather, a leftist revisionist.
After the US left, ARVN got the equipment left behind. They had the 4th largest Air Force in the world.
And legendary graft.
US aid wasn't cut off but it was reduced from $1 billion to $700 million in 1974. That shouldn't have been enough to cause their collapse in 1975.