Yes China cannot control how other people sees it, but what China have a large say in the fate of future Korea, unification or no unification.
Therefore its NOT in China's interest to have a unified Korea, just as it was a mistake for China to unify Vietnam.
ps.. the anti Chinese hate in Korea is very widespread, its not just those crazy brainwashed religious groups that you think.
I agree with most of your thought except that I think you are giving the "North Vietnamese betray" too much weight.
Let's revisit the history. Vietnam under various names (more accurately regimes or kingdoms in that part of land) had always struggled to expel the control of the various Chinese dynasties since day one. The day was 2000 years ago when the first state in that area was established by a Chinese general with local populations. Very often, the local rulers would try to gain acknowledgement from the Chinese emperors when fighting other factions for the Vietnamese throne or dominance. But right after they consolidated their control internally, they try to exclude influence and control from the emperors. They would even secretly style themselves as "emperor" which is a declaration of equalness with Chinese emperor, that is in modern term full independent. They only maintain the superficial obedience by addressing themselves King in correspondence with Chinese officials.
All this is to say that the recent "betray" is nothing new and China has experienced that many times but made the same choice anyway in the past. That should make you think why. After knowing the why, you may reach the same conclusion as the Chinese leadership made in the 1960s.
A hint to the thinking is "Sino-Soviet rivalry in 3rd world countries". If China did not support NV, Soviet will. If NV failed, US will stand at the Chinese border. If NV wan, USSR will stand at the Chinese border. Either way, China will be facing one of the super powers at her doorstep,
nightmares. Of course, after the breakup of NV with China, China was facing a Soviet puppet at the door, but only a puppet not Soviets itself, big difference.
The point is at least
at the moment of decision (1960s) China tried to play to China's favor, if succeeded (NV not breaking) the outcome would have been great, if loose (as what actually panned out), the outcome is
not any worse than either super powers at door step. This is to say that China made the only logical choice back then.
One can not judge a historical event or decision in the past after the event has panned out. There is no "what if" in history. So using the Vietnam case as "bad" example is not relevant to the future Chinese position about Korea.