Why did North Korea and north-central Vietnam belong to China for nearly a thousand years, but they didn't speak Chinese?

Almost all of Korea belongs to the Han Dynasty, and all of Korea belongs to the Tang Dynasty. North-central Vietnam did not leave China until the Song Dynasty.

The main reasons why they don't speak Chinese are:

One: the government does not immigrate to this area. The government plans to immigrate to the northwest region bordering nomadic people, and the two places have never immigrated.

Two: folk immigrants go as far as Guangdong and Guangxi in the south and rarely go to Vietnam. North Korea has some, but it is also dripping into the sea.