I've read what little I could find, in my studies of the modern Chinese military, on the Sino-Viet border war of the late 70's and from what I've gathered the Chicoms were beat the hell out of by a smaller, more lightly equipped, highly foot mobile, Vietnamese unconventional force. Think of it as an Asian jungle version of the jaw slapping the Finns gave the Russians during the Russo-Finnish Winter war of '39-'40, but on a much smaller scale. the Viets destroyed perhaps as many as 200 Chinese tanks but lost a sizeable northern city before the Chinese pull out. (don't rember the name) In short the Chinese didn't count on the fierce resistance of the local Vietnamese militias and shortly pulled out after about a month and a half or so. There were many smaller skirmishes through out the '80s and another rather large engagement took place around '89 or '90, I think, over some of the Spratley & Paracel islands, if I remember correctly. These islands will be fought over again in the future as China gets more bold. the Spratleys and Paracels are claimed by China, Vietnam, Phillipines and Singapore.
This rough experience was a catalyst for Chinese modernization of it's elite forces, small unit tactics and it's small arms development and many say, resulted in the Type 81 series of rifles.
From what I can gather, both the SKS and AK were used by both sides. The Vietnamese were much more effective in their deployment of their small units and weapons. It's my under standing that the groups that were mainly employing the AKs used them in short range hit & run drills and really liked hand grenades, while the SKS equipped (usually the local militia/and there were also quite a few moisin-nagants in the mix) would engage in long range ambushes and defend & delay actions from key terrain features, in order to buy time for the people in the built-up areas and in the hope that the Vietnamese gov't would finally come in strength, which they did not.
I'm sorry if this was long winded and probably didn't cover the small arms aspect well enough but this is a very difficult subject to find intel on. I hope this helps a bit.