Korea was mostly sinicized by 200 AD, when Goguryeo finally managed to retake territory in Manchuria, yet it had been under Chinese rule for about 300 years, and had absorbed its influences. It had been introduced to chinese characters somewhere before that time, as there is evidence of them during Gojoseon, which ceased to exist in 108 BC, and facilitated communication between Korea and China, and later with Japan after Baekje introduced them somewhere between 200 and 400 AD.
If the Han or the Tang had managed to incorporate all of Korea as a part of their empires instead of just keeping Manchuria, then the Koreans would have most likely have become a minority, although it would have wasted a lot of resources, as although Gojoseon only resisted the Han for about a year, their troops were strong enough to actually push their enemy back for a while. In fact, even though the king had died, the palace still remained until several sieges finally brought it down. Later, Goguryeo pushed out 3 million troops and contributed greatly to the downfall of the Sui.
The Mongols would also have had a hard time completely annexing Goryeo as a province, because there were about 7 invasions that were all repelled successfully, and the Mongols could only manage to control it as a tributary state. Of course, Goryeo contributed troops to the Yuan when they attempted to invade Japan, but that was required as a tributary power. When the Ming finally crushed the Yuan, they recongized Korea as an independent kingdom, although they still demanded tribute. The same went for the Qing.
By the Imijin War, it would have been way too late for China to attempt to annex it as a province. The Ming were also effectively wasted, as they made extensive efforts to make sure that Korea would not fall, leading to an invasion of the Chinese homeland and a lengthy struggle in East Asia. If Yi Sun-shin had either been put to death or had not existed at all, then it wouldn't have taken Japan long enough to make Korea into a colony. As a result, I could see Japan making Korea into a colony at that time, but probably not China doing the same.
The Yunnan might have been independent for about 500 years because they had natural borders which prevented China from conquering them effectively, but when the Mongol Empire came into power in China, a traitor somehow betrayed his country and it was eventually conquered in the 1200's, but it wasn't until the Japanese invasion in WWII that mass numbers of Chinese started to migrate to the area. However, the Chinese had already started to exert their influences by giving civil service examinations which required the use of chinese characters.
Does that help?