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.