Soviet national interest in NVN was opportunistic, and the Red Army was not a power projection force.
Without the port of Haiphong and with the Red River dykes breached and Hanoi underwater, there would have been no dry RPGs to send south. The VC and NVA in the south would have been reduced to survivalism and “true” guerrilla warfare living off the land, if they chose to stay and fight.
It was fully supplied NVA conventional forces that won the war, not the VC.
No beans and bullets, no war.
In response to your question why we chose to fight on the strategic ground chosen by the communists — we had a lot of foolish people in the White House who believed that the lessons of history didn't apply to them. Some things never change.
The antiwar, pro-enemy forces had years to organize and succeed because Johnson was gutless and dragged the war on and on with his limited war-nation building strategy. Some things never change.
You are correct about he the unsuitability of terrain for conventional forces in Vietnam. I went back to visit Vietnam with my wife in 2000 and an M-48 tank that got mired up to its fenders while I was there in '66 is still there, exactly where we left it. That mud is like glue.
You still sound like the Air Force. i.e., "we still could have won that thing if we bombed more". I don't agree. Bombing does have an effect but it is not decisive. Resilient people just dig in and work around. When the trucks were interdicted, they used pack animals (including elephants), bicycles, and increased sea transport. After the war, we discovered that they were transporting a large percentage of war materiel via coastal shipping despite our navy blocade. By the way, the idea of breaching the Red River dikes was bruited about by Jane Fonda in 1971 as an example of our war criminality. She was lying, as always: we were attacking the POL loading facilities on the dikes -using conventional bombs - a heavily defended and legitimate target, not trying to "flood the Red River basin to cause the genocide of the Vietnamese people" as she said then.
You underestimate the commitment and involvement of the Soviets and the Chinese during the war. They invested Billions to ensure that the "National Liberation War" in Vietnam was successful. The Soviets infused the area with state-of-the-art antiaircraft systems and advisors and the Chinese also had advisors in North Vietnam and even in the South. One of our snipers killed a Chinese advisor as he was standing in a sampan near us. The ring he was wearing was big enough to slip easily over my thumb! (I didn't get the ring).
The Soviets and the Chinese were heavily invested in a communist victory and we would have seen the Chinese ground forces if we had landed in Vinh as planned. The Chinese PLA is primarily foot-mobile and they have lots of soldiers to expend. As I am sure you remember, they were very competitive with the Soviets for the leadership of the Communist world at that time, so I believe that the threat of Chinese intervention was very real.
The pro-enemy/"antiwar" organizations were led by old-line communist cadres supported by the Soviets and later directly by the NVA. At the top of the People's Coalitionfor Peace and Justice and the New Mobe you will find Irving Sarnoff and Dorothy Healey and Bettina Aptheker and many, many more hidden faces who corresponded with the enemy throughout the war. They received money and direction and followed their lead. The press (like today) camouflaged the whole movement as a "spontaneous reaction to an unjust war" but it wasn't to the movement's leadership. They knew exactly what they were doing and the NVA called the tunes from the timing and placement of demonstrations to the "thrust" (theme) of each event. The FBI knew all of this but no Attorney General inconvenienced anyone. Interesting, isn't it?
And subvert the US media. They were a key element in our political loss of the war. The VC had been defeated. The NVA opportunistically invaded after almost all of our forces were gone, and even then, that took a couple of years before they did it.