Which factor most likely accounts for the greater influence of China in Vietnam than in other parts of Southeast Asia?

The region that occupies Vietnam today was dominated by China for eight centuries: domination began 100 years before Christ, and historians claim that it ended in 938. The periods of Chinese colonization were four in that eight centuries, during which, in addition to having a Chinese government, and establishing the Chinese language as that of the Vietnam region, Chinese techniques were used in architecture, to improve rice crops, and Chinese techniques for animal husbandry were also used. This undoubtedly left a profound influence on Vietnam more than any other culture.