Vietnam's Klub-equipped Kilo submarines are a dangerous combination for China:
China is making angru noises to the UN, Vietnam and Russia about the little publicized Russian sale of Klub submarine launched cruise missiles to Vietnam. China wasn’t happy about Russia selling Vietnam six Kilo class diesel electric submarines in 2009. Russia and Vietnam were quiet about the sale of 50 Klub missiles but the news eventually got out, in part because 28 of the Klub missiles have already been delivered, along with three of the Kilos. Another two Kilos are being delivered in 2015 and the last one will be completed in 2016 about the same time the rest of the Klub missiles arrive. Vietnam is one the many nations in the region threatened by Chinese claims to most of the South China Sea and given the long (over a thousand years) hostility between China and Vietnam, there is understandable fear that, even in defeat, Vietnam would use Klub missiles for one last attack on China.
While increasing the cost of aggression with these weapons is a good policy to deter aggression, the military capability that provides that increase in deterrence is counter-productive if the stronger power to be deterred has it within their capabilities to destroy or significantly degrade that smaller power's military capability:
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.
For poorer countries, the investment to protect their assets is often a penny-wise but pound-foolish failure too easy to make. The shiny subs or aircraft look spectacular. The protected submarine or aircraft shelters are less sexy (as are the maintenance and logistics support to use the subs or planes for long, for that matter).