Something to also note, back then and even maybe a bit now. The infrastructure, e.g. roads, rail, etc., in many places along the border was not that good. And in some locations the terrain was very restrictive for vehicular movement. So certainly that would be a consideration to fight a conflict along the borders. And if the campaign was going to penetrate deep into enemy territory.
Objectives, like Bastogne during the Ardennes Offensive.
Controlling road nets, etc. would be critical. So capturing the few villages, towns, etc. that influence road/rail movement.
As far as the effect on the US War in SE Asia. In certainly would impact the supply to the NVA/VC of all classes. By both their biggest benefactors, and fellow Communists.
However, how much it would effect the NVA/VC efforts to get the US/SEATO out of their country. Not sure, but it was NVA's/VC's backyard. They really didn't have to do anything. More then continuing inflicting losses on the ARVN/US/SEATO. And they really didn't have to "win" … just not "lose".
But without/limited supply/influence by the USSR and PRC. The US may have invaded the North and drive on Hanoi ? But that is purely speculation on my part.
Type 69 – the PRC would need everything they could produce taking on the USSR. Whether they would push to produce better equipment of all sorts. They might just go with their "enemies'" philosophy – "Quantity has a quality of it's own", IIRC the "saying" …
Even outnumbering the USSR. Every citizen in Chine old enough to carry an AK, ammo and a sack of grenades. Would be a de-facto member of the military. And/or be a huge Guerilla army. If the USSR decide to invade.
Of course if WMDs were employed all bets are off. It would again come down to numbers. Who would have the most survivors after the exchange(s). And if USSR WMD tech better than the PRC's. And again the numbers of those WMDs available.