I think the main problem of China right now is US pivot. The other regional problem like Vietnam and Pinoy are less significant for China. but, both problem linked together and potentially cause bigger problem to China. So, the best strategy for you is to disturb the Japan - Vietnam - Pinoy alliance. Because if this alliance disturbed, the US pivot will also be disturbed. And we know that Vietnam is the weakest part of this alliance, because Vietnam is just like you. Both are communist government.
If Vietnam goes into chaos and become a fully pro west, China will get hurt in geo political battle against US. So, with borrowing an old Chinese teaching that I know from my father (I don't know how to write it it English and Chinese) If you chase the small prize, you'll lose the biggest one. I think China should make a deal with Vietnam Government. Sometime you must sacrifice small matter to win a bigger challenge. That's why greedy people always lose.