"Victory goes to the strongest battalions" - - Napoleon.
Here with the REAL REASON for American and South Vietnamese defeat during the Second-Indo-China War.
Those "reasons" as enunciated by Dayan [thanks to van Creveld] and Truong valid as stated but immaterial as to the REAL REASON?
The REAL REASON the communist forces emerging triumphant in the classical manner of warfare  due to:
* Divisions of tanks. [armor heavy formations]
* Divisions of artillery. [massed units of artillery]
* Massed divisions of tanks and massed divisions of artillery operating in concert.
Those massed and prodigious formations of tanks and artillery operating in concert creating an inexorable force used in a prodigal way, CONCENTRATED AND FOCUSED, almost literally blasting a path for further advance and annihilating those South Vietnamese army combat arms units [ARVN] willing and able to continue the fight, the ARVN not having the physical wherewithal to combat such an adversary in the first place, also in time losing as well all moral and mental capacity to do so unit cohesion disintegrating!
From the time of the communist Easter Offensive  and even more so during the final push on Saigon  those South Vietnamese army units [ARVN] faced with an adversary employing armaments for which the ARVN had NO answer. THAT SOUTH VIETNAMESE ARMY HAVING BEEN ORGANIZED, TRAINED, EQUIPPED TO FIGHT THE SAME ENEMY THE U.S. ARMY FACED IN VIETNAM [light infantry of the North Vietnamese Army], THE ARVN FINDING THEMSELVES AT A DISTINCT AND OVERWHELMING DISADVANTAGE, NO AMELIORATION OR REMEDY TO THE PROBLEM POSSIBLE!!
[it has been suggested that the Tet Offensive of 1968 was a clear indication of American intelligence failure in Vietnam. Perhaps an even greater failure was the additional failure of American intelligence to detect and understand the very MASSIVE armor and artillery build-up that preceded the Easter Offensive of 1972! That those massive formations of tanks and artillery were not observed in some fashion is indeed a mystery not ever fully explained?]
That Republic of Vietnam [RVN] government in Saigon unable to deal with the situation, American air power withheld, the troops on the ground [ARVN] finding themselves vastly outgunned and without support or help on the way, collapse AND ULTIMATE DISINTEGRATION was not only a matter of time but inevitable.
Overwhelming force used in an overwhelming manner "poor intelligence" "hearts and minds", "ineffectual government" [Saigon] and "privileged sanctuaries" small potatoes compared to the REAL REASON for the final outcome.