In the post-classical period (600 CE–1450 CE), Vietnam and Korea both contended with Chinese territorial expansion and culture. They differ most in how much Chinese culture and influence they adopted, through a process known as sinification.
During the Tang Dynasty in China (618–904 CE), emperors led conquests of their surrounding regions to increase Chinese territory and to create tributary states out of their neighbors. In addition to Japan, Korea and Vietnam became tributary states. Korea and Vietnam, however, sinified to two different degrees.
Korea adopted more Chinese culture than Vietnam. Korea adapted Chinese script into its own written language, borrowed technologies, set up its government and education systems to mirror China's, and even sent its young boys to study in Confucian academies in the Chinese capital, Chang'an. As Korea is so geographically close to China, cultural transmission happened efficiently through trade.
Vietnam sinified less than Korea did. In Vietnam, unlike with Korea, we see a more Buddhist influence. While Chinese Confucianism caught on in the north, below the swamps and mountains, Buddhism and local religions were more prominent. In addition, Confucianism seemed too strict to adapt to the Vietnamese social lifestyle, where women enjoyed more freedom than their Chinese counterparts and where family units were structured differently. Chinese cultural transmission seemed more of an attack on Vietnamese culture than it did with Korea. Vietnam also had a more distinct, individualized culture. Other ethnic groups were encouraged to intermarry with the Vietnamese, including the Khmers (from today's Cambodia) and the Tais, suggesting that Vietnamese society was more independent from China than Korea's society. Additionally, given geographic and cultural differences, the Vietnamese had formed drastically different cultures, differing in everything from style of dress to art and literature.