Both USSR and Vietnam showed great restraint, avoiding an escalation of the war. there are many things you don´t know. the USSR amassed the largest ever troop concentration along the common border to China. A Soviet naval task force was dispatched to Vietnam, supplying warships, tanks, ammunitions and everything else. including 150 fighter jets. there is little reason to laugh on USSR-Vietnam military alliance.
true, Vietnam leadership learns entering a military alliance is a grave provocation for China. Not the lame excuse of Cambodia. in reality Chinese don´t care of. even if the Cambodians killed themselves until the last man and women. yes, that is why Vietnam is very hesitant towards any alliance in the future. Although you should not see it as a weakness and invitation for arrogance and aggression.