Let's put this way:
1/ U.S, Russia, Nato never want China as a super power and no countries want to be lower level. If there is a chance to let 1 of them down, the other will maximum support to get rid of the other.
2/ Let's say U.S fully support for Viet Nam weapons to fight with China, what will U.S loose? U.S will clear of it debts, totally control Taiwan as it pet.
3/ Let's say Russia fully support for Viet Nam weapons like it was, what will Russia loose? Russia got a sweet revenges due to the sucks of copied Russia product. There for the border clashed between 2 countries and Russia love to get rid China for best. The World only remain 2 tigers instead of three.
4/ Let say's NATO fully support for Viet Nam fight to China? What will Nato loose? The investors from Nato nations can move there business to ASIA, or Africa due to cheap labor and not to worry about "Copy" China.
5/ Let's say Viet Nam head to head with China? What will Viet Nam loose? Viet Nam as standing as poor country, that also mean with or without war, Viet Nam still step by step to built up from Infrastructural to the highest modern technologies. Which is cost Viet Nam more money spends to built than starting from scratch.
Caused China attack Viet Nam first, self-defense that a positive count for Viet Nam. Its does not need to pay money for investors who was invested in Viet Nam. All Chinese company invested in Viet Nam will be stuck in the middle of nowhere. Viet Nam could attack all China majors cities which are close or meet the exceeding missiles range.
This war, might a war to tear China into pieces, China has so many problems with Tibet, Xingjang, Inner Mongols. Manchu and Taiwan. Once China busy fight with Viet Nam, there is more chance for the new independence states grows.
6/ What China got if it win a war? China will pay for the investors who invested in China period, China may control all Islands for temporary but It will not safe for China to keep them forever.
China got sanction by all nations, all Chinese who lives outside will got discriminated by the nations. Chinese in Mainland will be staving for death.
What if China craziest to use it nukes? The whole nations who has nukes capable will drop China gone with the winds due to countries signed of NTP.