well, obviously we have two ways of doing this, speed Soviet involvement or delay US exit, preferably both at the same time
delayed US exit. Hmmmmm. French do better but still loose (or at least have to pull out and recognise Vietnamese independance). So, better campaign that somehow removes part of VM leadership. VM accepts partition plan ("temporary"), French pull out of north but retain presence in South. Slowly they decolonise Vietnam (focusing on Algeria instead?). sensing weakness North decides to have a go at unification, VC campaign in South increases, US sees this as a threat and decide to help, OTL style. Sino-Soviet split is really complicating matters, specially as Chinese involvement in A'stan increases. Soviets send aid to North but less then OTL. Camapign drags on, results in massive Tet-like uprising which is somewhat successful even military wise. US decides to call it quits, packs and leaves. Mid 1980s? OK.
Soviets invade a few years earlier. maybe mujahedeen are more successfull, amybe A'stani CP infighting is worse, maybe Iranain revolution happens a bit earlier, anyway, Soviets are in by mid-1970s, OTL scale. US sensing they can cause troubles for SU, returning the favour for Vietnam, send aid. chinese, as part of thaw in Sino-US relations decide to do it as well, sending it through Pakistan. War drags on for a decade, in mid 1980s Soviets decide enough is really enough, launch massive, country-wide offensive, declare victory and pull out. few are fooled by such declarations, they realise SU couldn't handle bunch of cave-dwelling muslims and troubles erupt in Stans. amssive soviet crackdown only causes simialr troubles to spring up from Caucasus to Baltics, SU collapses in early 1990s but truckloads of regional conflicts erupt on it's ruins.
how's that?
EDIT: Vietnam and A'stan could be neatly tied together with both sides offering to cut aid to other side's enemies in exchange for their respective pull-outs. Unofficially of course.