North Korea is not a Chinese vassal. The only way China can get NK to relinquish nuclear weapons is to place NK under a Chinese nuclear umbrella, which wouldn't even be feasible since China doesn't have a first strike policy. Without nuclear weapons, NK would be at the mercy of US and SK, and since China has a mutual defense treaty with NK, this would be another pressure point the US could exploit.
The advantage of a nuclear NK is two-fold. First, NK secures China's northeastern flank against US, SK, and Japan. A nuclear NK is China's first strike card. In the extreme eventuality of NK nuclear attack against Japan, China can still de-escalate by sacrificing NK. Trading NK for Japan would be more detrimental to the US than to China. Second, a nuclear NK can reduce its defense expenditure on conventional military, enabling it to develop its economy. A divided Korea is not indefinitely sustainable, so there needs to be alternative options to a US-aligned SK-led reunification. One is a non-US-aligned SK-led reunification. That was the option China tried to pursue 10 years ago, but I don't think anyone still believes that to be viable. The only other alternative then is a NK-led reunification, and that is only possible with an economically strong NK.
