I've answered this previously in this thread.

The period we're going to speak about is completely focused on conventional warfare, as opposed to the asymmetrical warfare in which the Vietnam War (the American portion of it) had largely been fought. With the exception of two major occasions, the Tét offensive in 1968 and the Nguyen Hue (Easter) Offensive in 1972, the People's Army of (North) Vietnam and the Army of the Republic of (South) Vietnam (henceforth abridged as PAVN and ARVN) had never truly faced of against each other in a conventional battle. In fact, one of the main reasons for the ARVN's failure in conducting proper counterinsurgency is that it had been trained since 1955 to repel an expected invasion from the North. When the time came for the PAVN to face off against ARVN in the Easter Offensive of 1972, the PAVN suffered several losses and was ultimately beaten back by the ARVN with help from American fire support. The PAVN had yet to perfect combined arms tactics and suffered greatly because of it. The ordinary ARVN soldier, who is usually the scorn of popular history on the Vietnam War, labelled as nothing but incompetent and coward soldiers, showed remarkable courage and fighting ability in fighting the Tét offensive (alongside American troops) and the Easter Offensive (practically on their own with American advisors or special forces).

So if the ARVN had managed to fight off the PAVN in 1972, what went so disastrously wrong in 1975?

There are several reasons for this.

The Americans had left Vietnam two years previously and while all equipment and machinery had been left behind for the South Vietnamese to use, they were practically useless without ammunition or spare parts - items which the US were not prepared to supply. While President Gerald Ford tried to gain support in Congress to increase the money given to South Vietnam, the ARVN found itself in an ammunition shortage. Since the US were not prepared to help South Vietnam with fire support like in the previous two conventional encounters, it would be an increasingly difficult task to stem the tide of North Vietnamese crossing the DMZ. The PAVN was also superior in numbers, having increased in size and improved itself during late 1973, but which had been constantly developing since 1968. PAVN had several veteran units, and plenty of soldiers in the PAVN had combat experience and were of rather high quality. Unlike the 1972 offensive, combined tactics training had been carried out and improvement on collaboration had been achieved. Combine this with competent generals and commanders in the field as well as sound and proper preparation for the offensive (in particular when it came to logistics and transportation) as well as the successful use of deception tactics to disguise that Ban Me Thuot was the target of the initial 1975 offensive.

The ARVN by this time was unfortunately still plagued with the corruption of senior officers and with widespread lack of proper training. However, when put to the test, the average ARVN soldier could stand his ground. To say that the North crushed the South instantly is perhaps too much of an exaggeration. ARVN stood its ground on plenty of battlefields, right up to the end at Xuan Loc. However, we have to consider the human factor in this and many soldiers feared for possible reprisals. Considering the importance of family in Vietnamese culture, it was only natural for men to desert to seek up their family amongst the refugees, but there are also plenty of ARVN soldiers who used the thought of protecting their families as their prime motivations in fighting.

In the end, it was simply too much for an already weakened ARVN. Without fire support, without the necessary equipment, spare parts or ammunition and with the enemy close to their families, it became too much for them to bear. After the fall of Ban Me Thuot President Thieu decided to evacuate the Central Highlands and effectively cut South Vietnam in two. The final collapse came soon thereafter.

To read more on this and the 1975 offensive, I'd recommend Gorge J. Veith's Black April: The Fall of South Vietnam 1974-75.

In pure numbers the Army of South Vietnam was of a similar size to that of the North. However the tactical position they were faced with in 73 could not have been more dissimilar. The North had good control of it's territory, an experienced army with a stable command structure, and a large groundswell of support in the regions it was moving into. The South lacked all of those. The distinction between North Vietnam and South Vietnam hides the fact that it was not two states at war, but a civil war, with civilians having mixed loyalties on both sides of the border*.

While by 1973 it is true that a large number of the communist aligned troops operating in South Vietnam were North Vietnamese the first decade of the conflict was characterised by South Vietnamese fighters on both sides engaging in irregular conflict in the areas south of the Cambodia-Laos border, and a somewhat more conventional war in the North-South border regions.

The result of this continuous engagement in South Vietnam resulted in a far greater number of casualties of in South Vietnam (both sides of the conflict) than in the North, leading to an Army that was accepting a far wider range of recruits (including actively courting non-Viet Cong aligned militia/warlords). This ongoing guerilla conflict also meant that by the time the Americans withdrew there were significant portions of South Vietnam that were not under direct control of the Government in Saigon.

*The 63 coup saw the end of any significant support for the Saigon Government in North Vietnam, but even before that the social strata each group drew their support from as well as the rhetoric by which they constructed their legitimacy meant that loyalists in the North were more likely to migrate/flee than leftists in the South.

It is not about the numbers. That was what went wrong in the first place by counting bodies as an expression of a military victory. You do not win wars by counting bodies.

It is about the connection of the people, the armed forces and their political leaders. The north was able to formulate a political idea that attracted more people to their cause than the south or the Americans were able to do. Their ideology gave direction to the violence. The people fought for a cause, for a better life, against a foreign occupation.

In the decade before, the Americans broke the connections between their political idea, the support of their own and the Vietnamese people and the way they used violence. Ridiculous amounts of force were used.

For the Vietnamese it escalated after the anti war movement caused the American government to "bring the boys back home" and switched to Air Power to project military force. After that there was no way of turning the events for the good of the Americans. Bombs from airplanes kill randomly. It prevents you from implying policies and winning the population over on your side.

Once the Army of South Vietnam was left to fight on their own, they were confronted with the impossible task of repairing the damage done by their American ally.