China is Vietnam's oldest and most feared enemy. China and Vietnam fought many times over many centuries. Old Vietnamese saying “Give a Chinaman a spear as a friendly gift for Tet, so he can kill fish and feed his family, and he will march south with it in the Spring to kill you and take your food from your family.”
Vietnam was glad to accept supplies, technical assistance (SAM crew training teams, etc) and supply transshipment permission from the USSR and Eastern Bloc from the PRC, but Vietnam would never have accepted large scale PLA forces.
Russia would never have started war with US over VN. It was much better for the Sovs and the PRC to use the SEA war to bleed us. For them, it was very much an excellent “war on the cheap.”
Example, when we damaged Sov ships during mining Haipohong harbor, Gromyko was questioned by Western media as to the Sov answer. It was basically, “Ya gotta do what ya gotta do.”
Sovs were far to busy trying to feed citizens to get involved or take action because of war in SEA.
Our failures in VN and WOT are based on our lack of political will to accept a reality of war. “A nation's traditional military forces cannot fully defeat an unconventional (guerrilla) army in the field. But that gurilla army cannot succeed without the huge logistical support of a foreign nation (or in case of current WOT, intercontinental Islamofacism.) Defeat or make the battle too expensive for the logistical supporter, and you defeat the guerrilla army in the field.
Don’t forget, the VC would NEVER have defeated South Vietnam alone. After their horrendous losses after the 1968 Tet Offensive, they were NEVER AGAIN a significant force in the fighting. North Viet troops infiltrating from North Vietnam replenished the ranks of dead and maimed Viet Cong. The VC WEREdefeated. I got to Vietnam after Tet, and the corpses that we searched for Intel after firefights were always NVA with paybooks and other NVA documents. The only problem we had with VC was the minsweeping that we had to do every day between the Fire Support Bases and the villes. The local VC would do that and not much else, and they didn’t do much of that.
South Vietnam lost due to a blitzkrieg from a lavishly equipped (by the Soviets and Chinese) conventional North Vietnamese Army that did not fear the US reintervening despite the pledge to do so at the Paris Accords due to the DemocRAT Congress cutting off the ARVN’s funding after Nixon’s downfall after Watergate. The Anti-war DemocRATS, led by Ted Kennedy and others seized the opportunity to pull the rug out from under the South Viets.
The US made many errors in the counterinsugency phase of the war. Despite that, it DID deal a staggering military defeat to the Communist forces, and gained time for the Viets to militarily stabilize themselves so as to resist any future aggression. It was the pusillinamity of the US Congress and the subversion of the anti-war left that forced the outcome in 1975.