China would muscle its way in anyway. It wasn't as if the other claimants haven't tried to garrison the reefs. Vietnam (both South Vietnam and unified Vietnam) got the shortest end by having some seized by force. Any earlier and better enforcement would require a lot of help from the US or Soviets, depending on who the country's backers are.
I'm not sure about the Soviet, but the (South) VietNam saw the "help" of USA already. And well, things didn't go so well for them.
Back in 1974 (after USA and PRC "normalize" their relationship), PRC Navy sent a total of 4 warships (though they are more like minesweepers) to attack Paracel islands (being held by RVN - (South) VietNam). On the other side, RVN had 4 real warships (cruisers and destroyers) with better firepower and detection. The battle zone is also within range of their Air force.
And no plane took off that day (it appeared that it is a "request" from USA), and the RVN Navy was still f*cked (A combination of cowardice, friendly-fire and a lot more).
With regard to unified VietNam, from the late 70s, I think that the Soviet only helped by selling equipments and weapons, not directly join in the conflict.