Why would the Chinese launch on the Viet Nam border? Viet Nam is ALREADY part of the hegemony.
Not like they needed to compel the Vietnamese to agree to trade terms or port facilities.
China and Viet Nam have about two thousand years of bad blood between them.
1) Vietnam is a Russian ally, not a Chinese ally.
2) China wants Vietnam's off-shore oil.
3) China's biggest weakness is that their troops have no actual combat experience. Better to test them against a small adversary first, get combat experience, and find out where they need to improve.
The Chinese invaded North Vietnam after the American war ended. The Vietnamese kicked their asses.
China and Vietnam have long-standing grievances with one another that go back centuries. One reason why Vietnam is more closely aligned to Russia rather than China. China even briefly invaded Vietnam during the late 70’s after Vietnam occupied much of Cambodia (a Chinese client state) in their war with Pol Pot and the Khmer Rouge. They managed to fight the Chinese to standstill that eventually compelled their withdrawal.