In advance of the Paris Peace Accords in 1973, the United States transferred a great deal of military equipment and supplies to the armed forces of South Vietnam. In Operations Enhance in May 1972 and Enhance Plus in December 1972, the US delivered (approximately):
And these figures don't include small arms and ammunition.
The reason for these two large transfers was that the Paris Peace Accords were anticipated to strictly regulate the transfer and replacement of military equipment to the South Vietnamese forces—vehicles and aircraft would only be allowed to be replaced on a one-for-one basis. The 1972 Easter Offensive demonstrated the North's capability to conduct large-scale conventional offensives in multiple military regions simultaneously, which meant the ARVN needed as much strength, at least on paper, as possible at the conclusion of the negotiations.
Now, because of this massive transfer, the South was able to conduct military operations in 1973 and fare relatively okay at holding onto a stalemate situation, as the Accords were almost immediately violated by all sides. For example, by 1974, they were the sixth-largest air force in the world on paper. However, the North, reeling from its losses during the Easter Offensive, resolved to largely regroup and planned its next major offensive, projected to begin in 1975. Similarly, to the South, they too would need to resupply and reform its forces. Warsaw Pact aid to North Vietnam in 1972 completely replaced their losses from the Easter Offensive, and increased to record levels in 1974. The battles of 1973 largely proved to be inconclusive, but by 1974, the PAVN were beginning to probe at the edges of South Vietnamese territory more pointedly, conducting more and more divisional-sized attacks. They continued to whittle away at strategic points, and causing irreplacable losses of both men and materiel.
However, with the 1973 Oil Crisis causing a severe shortage in fuel, plus the ensuing economic recession suffered due to the crisis and the withdrawal of American forces, the South Vietnamese military suffered greatly. Furthermore, the South Vietnamese dependency on the United States for their supply chain of ammunition, medical supplies, vehicle and equipment spare parts, and fuel proved to be problematic, as military aid was continually slashed by Congress.
By mid to late 1974 and throughout 1975, the ARVN was forced into a dire situation—no fuel to operate its aircraft, trucks, and helicopters, which handicapped the ARVN who had been trained to fight a "rich man's war" with distinctly American style tactics of air mobility, mechanized infantry, and overwhelming artillery support. By the final days of the North Vietnamese Spring Offensive, South Vietnamese artillery was being rationed to six shells for 105mm pieces, four for 175mm pieces, per gun, per day. Hand grenades were strictly rationed and infantry had their small arms ammunition load reduced by 50%. In some cases, like in MR I, soldiers were being given a single hand grenade and 85 rounds per month. Additionally, medical supplies were so lacking, bandages were being reused. It didn't matter that on paper, the ARVN had tanks, APC's, trucks, helicopters, and aircraft if they did not have the fuel, spare parts, or ammunition for them. To add insult to injury, the acquisition of MANPADS by the PAVN meant that the VNAF could no longer operate with impunity within their own borders. Dozens of helicopters and aircraft would be lost to them, not including losses from conventional AAA. These factors in addition to problems of morale and strategy by the South, would contribute to the relatively quick collapse of the country in 55 days following the start of the offensive.
For further reading, I highly recommend Black April: The Fall of South Vietnam, 1973-75 by George J. Veith and Vietnam Combat Operations 1972 - 1975 and Vietnam from Cease-Fire to Capitulation by William E. Le Gro.