The reason is quite simple: history always repeats.
If vn wants to challenge China in SCS for a war, you will get a war. At the end of the day, you will get what you ask for.
One thing for sure is that it will not be China who will be left with ruins. If you still do not believe that and still think vn has the strongest and best army, well, there will be nothing else I want to add here.
In 1945, we were using bamboos, sticks, knives, etc against American heavy artillery, planes, jets, etc and kicked its a$$ out of Vietnam. Today we use heavy artillery against Chinese's artillery. One megaton bomb drop in Shanghai will kill more people than 1 megaton bomb drop in Hanoi. Not to mention we got allies such as India, Philippines, Japan, etc. You cannot bomb all of Vietnam when you leave yourself vulnerable to our allies. Do the math. You have no chance.