I have always assumed relations are somewhat sour between unlike the relations China enjoys with North Korea.. Vietnam is little bit alouf and stubborn for China's liking.. Am I right in my assessment
This is very complicated, I may have to explain a lot.
Let's look at the geography of China. To the north is the Siberian ice sheet, to the northwest is the great desert, and to the southwest is the highest plateau in the world.
In ancient times, there were only two channels for other countries to attack China, one from North Korea and the other from Vietnam. Therefore, for thousands of years, these two countries have been controlled by China for a long time. They are China's vassal countries.
But in the Confucian world view, control means not only rights, but also responsibilities and obligations.
The binding of rights and obligations is very important in the Confucian system, even many Chinese emperors would rather destroy their dynasties than subvert the Confucian system. Because these emperors thought that if the dynasty perished, a new dynasty would be born. If the Confucian system perishes, so will the Chinese nation.
In April 1592, the Japanese ruler Toyotomi Hideki invaded Korea. The emperor of the Ming Dynasty sent General Li Rusong to help North Korea defeat the Japanese army. Although the emperor knew that the war would seriously worsen the already poor financial situation of the Ming Dynasty, the Ming emperor chose to participate in the war. In 1627, 35 years later, the Ming Dynasty triggered a peasant uprising due to the serious financial crisis. Finally, the Ming Dynasty perished in this rebellion.
In 1883, France invaded Vietnam. The Qing emperor of China knew he would be defeated, but he still participated in the war. In September 1894, the modern Japanese army invaded Korea. Although the emperor of the Qing Dynasty knew he would be defeated, he chose to join the war. These two wars also indirectly led to the demise of the Qing Dynasty.
Now you should understand that under the Confucian system, China is bound to send troops to protect Vietnam and North Korea (in fact, if we have other options, we are not willing to go to North Korea to fight the Americans in 1950), otherwise, it may subvert Chinese tradition.
However, China does not allow other countries to invade these two countries, which does not mean that China will not beat these two countries. In fact, China has sent troops to destroy these two countries many times in history, which is why Vietnamese hate us.
At present, the relationship between China and Vietnam is not good. The Chinese hate the Vietnamese for betraying China and taking refuge in the USSR. In order to maintain diplomatic relations with the three Mekong countries, the Chinese government deliberately stays away from Vietnam(Vietnam had a war and border conflict with Thailand and Cambodia. Vietnam was prepared to annex Laos, and Laos is also very wary of Vietnam). However, Vietnamese people have too many Chinese blood and cultural ties after all, China will still help them in economic and other aspects.
In fact, North Korea's reputation in China is not good. Although Americans call China an dictatorship country, but except for Mao Zedong, all leaders of the PRC has been in power for no more than 10 years, and they have no kinship or blood relationship with each other. These bastards in Korea are clearly feudal dynasties, but they are holding the same flag as us. Not only drag down our reputation, but also let our soldiers waste their lives. Our soldiers died for communism, not for the feudal dynasty. But based on the Confucian system, we will still protect North Korea. It would be great if North Korea could reform its politics.