The conception of the war as I've described is something that had always seemed obvious to me and the idea of 'Vietnamisation' being anything other than a farce, ridiculous.
I've been wondering though, this view of events casts the North Vietnamese as the Goliath force and the Sourh Vietnamese as the helpless victims who without the support of their western allies never stood a chance and who, when the support was no longer forthcoming, buckled almost immediately.
But why didn't the North Vietnamese require the overt presence of USSR advisors and regular fighting forces and air support to be able to achieve reunification under their banner in a timescale drastically shorter than the war until that point had dragged on for with all the support for South Vietnam had enjoyed until that time?
I understand that there was a great deal of material support from allies of North Vietnam, was it comparable in scope to that received by the South though? Was it really a near certainty that the events that followed would unfold as they did?
But why didn't the North Vietnamese require the overt presence of USSR advisors and regular fighting forces and air support to be able to achieve reunification under their banner in a timescale drastically shorter than the war until that point had dragged on for with all the support for South Vietnam had enjoyed until that time?
The reason for this is because the North Vietnamese was more than capable of committing large scale offensives on their own without having Soviet or Chinese advisers present. During the First Indochina War, the forerunner to the People's Army of Vietnam, the Viet Minh, had extensive experience with Chinese advisers who were instrumental in turning a guerrilla army into a conventional army. By 1975, there were very capable North Vietnamese generals, some of whom had experience reaching as far as WWII.
There are several reasons for why the war took so long out of a North Vietnamese perspective:
The war began and was created to be that of an insurgency. It was supposed to mimic the First Indochina War and follow the Maoist theory of guerrilla warfare (three stages: preparation, guerrilla warfare, conventional warfare). If North Vietnam simply invaded South Vietnam in 1960 without any further ado, it would reflect very badly onto North Vietnam and make them out to be the aggressors. By letting South Vietnam be responsible for their own liberation, led by North Vietnamese military personnel, it would make the insurgency look like a natural reaction to the repressive South Vietnamese government. The North Vietnamese adaptation of the Maoist guerrilla theory placed a weight on propaganda and world opinion as something that had to be won over and portraying the southern insurgents as a rag-tag guerrilla force made up of regular young and old people fighting with antiquated weapon was bound to create sympathy the world over.
The PAVN was still being modernized. It was a conventional army that had continuously been modernized through the 1950s yet it wasn't until the American intervention in the Vietnam War that this became a pressing issue. The PAVN suffered several setbacks in the semi-conventional period in the beginning of the war ('65-'66) and additional setbacks in '68 and '72. These setbacks provided the PAVN with valuable lessons for the future however. It is also important to emphasize the Chinese and Soviet contributions to this modernization: North Vietnamese troops were trained in both countries and instructed in the use of modern weaponry. The material support by China and the Soviet Union to North Vietnam was most certainly comparable in scope of that of the US to South Vietnam. Soviet advisers served in capabilities dealing with electronic warfare and advanced SAM sites while the Chinese contributed a total of (at its peak) 16 anti-aircraft divisions to serve in North Vietnam, and according to Chinese statistics were responsible for shooting down a total of 1707 American planes. Other Chinese troops that served in North Vietnam were engineers to help rebuild railways and communication lines. Both the Soviet Union and China were firm in not allowing their troops to cross into South Vietnamese territory, seeing more value in having the soldiers defend North Vietnamese territory if it would be invaded.
The American factor is important. North Vietnam did try to conquer South Vietnam by a semi-conventional offensive in 1968 and by a full-on conventional offensive (with T-54/55 tanks ) in 1972. Each time it failed. Why? Because conventional warfare was the American forte and overwhelming firepower stopped the North Vietnamese each time, although the PAVN committed mistakes (such as weak co-operation and lack of proper training in combined tactics). When the United States were gone and without the prospect of them being able to supply South Vietnam with any more war materiel, the stage seemed to be set for the final offensive - an offensive which was actually planned to last until 1976 but due to events on the battlefield managed to end much faster. See my thread on South vs. North 1975 here.