Vietnam is getting land-attack cruise missiles for their new submarines:
Vietnam is in the process of acquiring 50 anti-ship and land attack 3M-14E Klub supersonic cruise missiles for its burgeoning fleet of SSK Kilo-class diesel-electric submarines, Der Spiegel Online reports. ...
The Klub is a Russian-made conventional supersonic cruise missile, “designed to destroy targets protected by sophisticated active air defenses and countermeasures,” deagel.com explains.
Just having these subs gives China the incentive to open a conflict with Vietnam by trying to Pearl Harbor the subs in port.
Mind you, I'm not saying that Vietnam is provoking China by seeking effective weapons to deter or defeat China.
I'm just saying that to prevent such weapons from increasing the chance that China will strike first, Vietnam needs to protect the subs that are in port to deny China the incentive of striking first in a crisis.