We had stopped arming the RVN, so they were not well-armed. While they had plenty of ineffectiveness issues, if we had supported them half as well as the Soviets continued supporting NV, they might have held on. At the end, they had squat in the way of logistics to support the arms we had left behind for them. No bullets/shells/bombs means the arms were worthless.
The North Vietnamese took out a massive loan from the Soviets to purchase the arms needed for the invasion in 1975. If we had provided a similar loan to South Vietnam, they might have held on. But Congress cut off all aid, including loans, to make sure that North Vietnam would win. Just how big was the loan? The Vietnamese hadn't paid it off 20 years later. They settled up with the Russians for lease payments the Russians hadn't been making for the Cam Ranh Bay facilities.