lol, "too late." Not even Japan can go nuclear in months, and you don't have the missile tech. Once you get it, Cambodia will get nukes hand-delivered- and we can say bye-bye to Hanoi within days.
For one, North Korea isn't trying to steal Chinese territory, doesn't claim three Chinese provinces, and doesn't have a freakish obsession with "getting back" at China for a war that ended 600 years ago (that Viet started)
Taiwan could wipe Vietnam off the face of the planet even without nuclear weapons. If you're so tough, why don't you take Taiping Island? Don't try your hand against China + Thai + Cambodia + Taiwan.