The Vietnamese benefited greatly from Chinas influence and rule. The Chinese exposed them to a more sophisticated way of thinking and introduced many new innovations and other technologies. They built waterways, harbours and roads in northern Vietnam. China also assisted in improving agriculture, by introducing irrigation methods, and other things that decreased manual labour. China did attempt to sinicize the people of Vietnam. However, the Vietnamese completely resisted any attempts.
Throughout it all, they maintained their own traditional culture and beliefs.
They adopted and used the benefits of Chinese civilization, while still resisting total assimilation. Later, sometime in the 1st century, China realized that allowing local lords to rule was inhibiting their attempt at sinicization. So to rectify this they imposed direct Chinese control. They continued attempts at sinicization even more forcefully. They suppressed all local customs, and in their place enforced Chinese ones. Chinese customs, religions, languages, and teachings were imposed by force. Even Chinese clothing and hairstyles became mandatory.
By the 15th century, when China had just reclaimed the territory, any attempts at sinicization made the Vietnamese resist even more fiercely.