Well, the professed goal of the 1968 Tet offensive,
according to the North Vietnamese themselves, was to create a general uprising among the South Vietnamese population. If your assertions were true that 90% of the population supported the Communists, then Tet 68 would have been the perfect time for this uprising to take place as the North Vietnamese hoped for. It did not however. In fact, in many areas it had quite the opposite effect:
"
Many urban dwellers were indignant that the communists had launched their attacks during Tet and it drove many who had been previously apathetic into active support of the government. Journalists, political figures, and religious leaders alike—even the militant Buddhists—professed confidence in the government's plans"
Aftermath
North Vietnam
The leadership in Hanoi must have been initially despondent about the outcome of their great gamble.[148][149] Their first and most ambitious goal, producing a general uprising, had ended in a dismal failure. In total, approximately 85,000–100,000 communist troops had participated in the initial onslaught and in the follow-up phases. Overall, during the "Border Battles" of 1967 and the nine-month winter-spring campaign, 45,267 communist troops had been killed in action.[150]
[171]
The keys to the failure of Tet are not difficult to discern. Hanoi had underestimated the strategic mobility of the allied forces, which allowed them to redeploy at will to threatened areas; their battle plan was too complex and difficult to coordinate, which was amply demonstrated by the 30 January attacks; their violation of the principle of mass, attacking everywhere instead of concentrating their forces on a few specific targets, allowed their forces to be defeated piecemeal; the launching of massed attacks headlong into the teeth of vastly superior firepower; and last, but not least, the incorrect assumptions upon which the entire campaign was based.[151] According to General Tran Van Tra: "We did not correctly evaluate the specific balance of forces between ourselves and the enemy, did not fully realize that the enemy still had considerable capabilities, and that our capabilities were limited, and set requirements that were beyond our actual strength.
[152]
What the North Vietnamese did NOT predict was the complete turn around in public opinion that Tet 68 would produce in the United States. Once the North Vietnamese recognized that most of the news outlets were declaring Tet 68 a disaster for U.S. troops and for our policy in Vietnam, the North Vietnamese quickly took advantage and declared the Offensive a victory.
Your assertion that the way the insurgency played out illustrated common support is also not factually correct. After Tet 1968, 1/3 of all Viet Cong forces had to be replaced with North Vietnamese Regular Army troops. So after 1968, 1/3 of all the "insurgent forces" were North Vietnamese Army regulars.
The horrendous losses inflicted on Viet Cong units struck into the heart of the irreplaceable infrastructure that had been built up for over a decade. MACV estimated that 181,149 Viet Cong and North Vietnamese troops had been killed during 1968.[156] From this point forward, Hanoi was forced to fill one-third of the Viet Cong's ranks with North Vietnamese regulars.[157]
This would indicate that the Viet Cong ranks were not filled by an abundance of South Vietnamese peasants dissatisfied and eager to fight after 1968 but instead a substantial portion of their ranks came from the North Vietnamese Army.