Both Vietnam and Thailand belong to China, right? Are they all separated from China? Like outer Mongolia, split up with China?

Thailand is not, but Vietnam is. Thailand was called Siam in ancient times. The earliest written record was in the Song and Yuan Dynasties. From the conquest of Qin Shihuang to the reign of Yang Di, North Vietnam has always been the territory of China. The Tang Dynasty broke away from it and became our vassal state. Conquered by France in the late Qing Dynasty, it became a French colony. After the founding of People's Republic of China (PRC), with the support of the Soviet Union, Vietnam officially broke away from China and became an enemy of China. Later, I was much more honest after being beaten.