The "bombed back to the stone age" is pretty much a myth. You have to have things to bomb that would cause supply issues, which the North Vietnamese really didn't have. You have to keep in mind, that North Vietnam was a 3rd world country, and could survive without modern conveniences. Pretty much all of their military supplies were supplied by the Chinese and Russians, so few factories are needed to make any of the supplies they would need. They also did not keep large stocks of supplies in one area, but distributed them around the country to help protect them against bombing. Bombing against the Ho Chi Minh Trail wasn't terribly effective either since almost all the infrastructure to help keep supplies moving was underground. There are thousands of miles of tunnels and shelters, hospitals, workshops, etc. along the trail to protect their people during bombings and keep the supplies moving. On the whole, the bombing campaigns against supply lines and infrastructure were ineffective. The bombing did hurt the NVA and VC in their staging areas and on the battlefield, but the North Vietnamese were willing to accept any number of casualties if it lead to ultimate victory. How do you destroy an enemy that is willing to sacrifice every man and woman to win?

The South Vietnamese military was very well equipped and well trained, (the NVA was too) but they still relied on the US to be resupplied for these things. But, probably the biggest problem with the South Vietnamese army was that they were very poorly lead, from the top down. Since the US pretty much took over combat roles from 1965-1970, the South Vietnamese leadership was not well experienced in combat against the NVA or VC. In addition to this, corruption played a major part in supplies not making it to the front lines in large enough quantities to be effective or they did not make it there at all.So if you're poorly lead and have no supplies to fight, what do you do? Run, surrender or die. This is what ultimately happened during the final invasion of the south.

Also, since Nixon had left the presidency, the North knew that the south would likely not receive any American air support, which is what they feared the most since the NVA was nearly annihilated by American air power during the Easter Offensive in 1972. The North eventually did come to the peace table, but the conditions in the Paris Peace Accords pretty much ensured that the war would eventually continue, but the Americans would not be there to help, which is exactly what the North wanted.

I hope this helps and isn't all jumbled up.. I'm not a good writer.

Something to bear in mind is the emotional turmoil of the war for the Vietnamese people themselves.

1: Sense of national pride National pride among Vietnamese could be built by preservation of the traditional heritage or independence from Western powers. While the North focused on transforming society, they did so slowly and focused on military power. The South was often run by leaders hostile to traditional beliefs and who were inclined to look to the West for economic, scientific, and military ideas. Failures to directly transplant military ideas were a visible loss of attractiveness of the Southern government.

2: South Vietnamese society was becoming something many Vietnamese didn't respect. Previously, professionals could earn a reasonable but not wealthy living. Since the massive influx of US troops and money, the black market and corruption became essential to survival. Prostitution became common, theft of government property routine, and politicking within the military was no longer required for advancement but for survival. Officers found themselves resorting to corruption just to feed their families on their own minuscule pay and draftees often had to choose between military service and ensuring that their family was fed. US money had raised prices for goods but supply and pay for government officials hadn't risen.

3: Murder threats were enough to dampen the energy of many soldiers. The real possibility of retaliation against one's family for special military service meant that regional forces simply did not feel safe operating as effectively as they could. The most effective ARVN units came from Saigon and part of that was the safety of the soldiers' families.

4: Fatigue was real. After so many years of killing and tension, many people just stopped caring and wanted this to end. That meant that if a politician was evacuating, everyone else thought the situation was risky and the motivation to keep on fighting was gone. ARVN officers of some bases turned their facilities over to NVA forces without even fighting. There were units who did fight at the end and could have continued fighting with air support but didn't have support capabilities to keep going on.

5: South Vietnamese forces were still in-experienced in high-tech roles such as aircraft maintenance. When the S pulled much of its support, South Vietnam was left with a lot of expensive weapons systems that got the bare minimum of repair and thus couldn't be fully used.

6: A conflict of priorities between long-term survival of South Vietnamese society and short-term military security was present and just became worse. Selecting senior officers often involved a choice between a brilliant but corrupt officer and an honest but unexceptional one. By the time of President Thieu, corruption was something the South Vietnamese government had agreed to work on and part of that was removing several corrupt officers and replacing them with more honest ones. Those changes sometimes produced gaps withing ARVN preparations that were exploited by the NVA.