The 1972 Easter Offensive was positively smashed by US Airpower, but also, ARVN units had gotten better. Airpower alone can't reverse an invasion.
But the invasion wasn't reversed, only halted. Even as it was, this halting took over a month. Additionally, the ARVN performance in 1972, while still better then in 1975, and was then followed by six more months of bloody stalemate where the South was unable to roll back the gross majority of the Northern gains. Although a few cities were eventually retaken, half of the four northern provinces was permanently lost to the Communists, and would prove a "dagger to the heart of the South" in 1975. As for the ARVN, it's performance noticeably declined from it's peak in 1970. So we can tell it was in the process of getting worse in the early-70s (or, to be more accurate, regressing to their norm without the influence of American advisors), irrespective of the foreign aide situation. American material aid never affected the rampant corruption, deep politicization, and subversion by the Communists that were the root reasons for the ARVN's decay. Only the subordination of ARVN training, logistics, and personnel appointment to American military advisors did that and all that was gone after the American withdrawal since it couldn't work without significant troop commitment.
Similarly, in 1975, had the Americans sent in airpower the PAVN would have gone to ground after it's initial success, but by then they'd have already been in control of the Central Highlands, which the ARVN would not have been able to reclaim, and would have rendered the South indefensible. It means that the moment the next offensive kicked off, which would occur once the Americans drew down their air operations like they did after the end of the '72 air offensive, the South would've fallen faster then the Americans could even react. That was the North Vietnamese plan. That the Americans didn't intervene with airpower was a welcome surprise for them.
As pointed out, RVN was short on POL and on some muntions, while the North had all what could be shipped into Haiphong.
In all supplies available to the northern forces, the quantities were only a fraction of what the ARVN was "short on".
Much is made on the ARVN having more gear and supplies than they could use, was partially true, they didn't have the fuel to run the supplies from depots to where it was needed.
Erm, no. The ARVN fuel supplies were adequate to meet their needs. The problem was administrative, namely that the ARVN's logisticians were too busy engaging in black market activities or fleeing the country to bother arrange shipping for anything. The lack of foreign aide may be a useful scapegoat for the reasons for the defeat of South Vietnam in 1975, but the explanation doesn't hold up when one realizes that the North Vietnamese achieved everything they did with even less.
Further evidence can be found in the fact that in the places where the ARVNs leadership
wasn't a bunch of corrupt political stooges, they gave the 1975 PAVN one hell of a fight.
Too illustrate the difference, we can look at two examples in the 1975 offensive of ARVN formations whose performance was on opposite ends of the spectrum: the 1st Division and the 18th
The 1st Division had a reputation of being the best in the ARVN after the successes of 1969/1970. This meant it was also the best equipped, best supplied… and also became the choice posting for the sons of the rich and powerful. Over the next five years, its formerly skilled officer ranks were replaced with pampered wannabe-generals. It was on the defensive perimeter around Hue when the Communists attacked. Despite outnumbering and outgunning the mere regiment attacking them, the division commander panicked after only 48 hours of fighting and surrendered the entire division, opening a huge gap in the ARVN lines, leading to the surrender of two other divisions and the rapid collapse of the entire defense of Hue. The Communists were so impressed with the readiness of the division to surrender that after only a brief 1 week "re-education" they put the 1st Division back to work moving supplies for their advancing Corps.
The 18th Division, by contrast, had a reputation as the absolute worst in the ARVN. It had formerly been the 10th, but was renamed as "Number 10" was Vietnamese slang for "the worst." This likewise meant it was at the bottom priority list for things like equipment, supplies, and replacements. The division had recently been assigned to the command of General Le Minh Dao. Dao was one of the best commanders in the ARVN, so good that the government identified him as a potential threat and shuffled him off to the 18th, which was basically a dead-end for his career. Dao whipped the 18th into shape, replacing incompetent officers, retraining the division, and rebuilding morale. The 18th was in the rear and took no part in the early battles around the Central Highlands, Hue and Da Nang - all disasters that saw the bulk of Vietnam's million-man army evaporate. By the time the Communists were advancing on Xuan Loc where the 18th had drawn up its lines there was little left to block the road to Saigon.
To date in the campaign other ARVN units had regularly folded in hours or days to smaller VPA forces, surprising even the Communists who had expected a stiffer fight. At Xuan Loc, the 5,000 men of the 18th faced an entire Corps of over 40,000. The Communists attacked, expecting an easy victory but were repulsed. They brought up more troops and attacked again, penetrating into the town before a series of ferocious counter-attacks again drove them out. They then pounded the town with heavy artillery and tried again in a masssive assault of tanks and infantry. After house to house and street by street fighting in which the infantry of the 18th knocked out dozens of T-54 tanks the VPA was again thrown back. When the 18th had been expected to maybe by a few days, it had now held for over a week.
The ARVN dared to hope that they might actually win, and plans were made to reinforce the 18th with the 1st Airborne Brigade, another of the South's elite formations. A massive helicopter drop was planned near Xuan Loc. Tragically the Ops Officer for the 1st Airborne sold the entire plan to the Communists, and the 1st Brigade dropped into one of the largest ambushes in history and was quickly annihilated.
With other ARVN units to the west breaking, half its strength casualties, almost out of ammunition, cut off and encircled with no chance of any reinforcement, the 18th's position had become hopeless. The VPA drew up six divisions around Xuan Loc and offered the 18th a chance to surrender. General Dao stalled through the day, and then in the night lead his the remnants of his division in a daring breakout to the south, punching through the Communist lines and escaping with his surviving troops to Saigon.
The bottom line is that the immediate reasons for the South's defeat in 1975 had less to do with the material situation and a lot more to do with poor political and military leadership which placed the ARVN in a very vulnerable starting position, and then proved utterly incapable of recovering after the initial setbacks.