So to me, it looks like the allies in Vietnam had two main threats to neutralize to win the war. They had to keep the Vietcong from taking over the countryside through insurgency, but they also had to prevent a conventional invasion and conquest of South Vietnam a la the Easter Offensive and the Ho Chi Minh offensive. Furthermore, it seems that the great materiel support the North Vietnamese were getting from the Chinese and Soviets was necessary for their victory, and that without materiel support to South Vietnam to balance it out, the odds are greatly stacked in the North Vietnamese's favor. South Vietnam just didn't have the resources to sustain a big conventional army on its own like you need to fight a big conventional army, and in trying to do so, the allies hampered their ability to fight the COIN war.
Even if the U.S. had the foresight to help make the ARVN a perfect COIN force, the victory would be only temporary if the NVA could infinitely regenerate itself with Soviet/Chinese materiel, since a perfect COIN force would struggle against a heavily armed big conventional army. Therefor, winning the war permanently would mean either a collapse of North Vietnamese will to conquer the South (which I think is unlikely without an invasion), or else the larger communist powers drawing down their support for the war in Southeast Asia. Is there a price they just weren't willing to pay to take over South Vietnam?
I'd really like to hear from people better versed on the communist side of the Cold War, as well as any Vietnam experts who can point out any misconceptions I have.