Isn't NPT already a piece of paper, though? North Korea signed, got nuclear technology and uranium supplies, developed a nuclear weapon, and then withdrew from the NPT. Iran signed the NPT, got nuclear technology and uranium supplies, and it's almost certain to develop a nuclear weapon. This is all thanks to Russia and China.
A lot of Chinese users here express outrage at the possibility of Vietnam developing nuclear weapons, but these same Chinese users seem remarkably forgiving about North Korea and Iran, and China's role in protecting these two regimes. It's fairly clear to outside observers that China would therefore not bomb Vietnam if it developed nuclear weapons, especially because of the message that would send to all of China's other neighbors.
In short, hasn't China greatly undermined the NPT? Why should China complain if Vietnam now violates the NPT?