For much of Vietnam's history the official written language was Classical Chinese, using, of course, Chinese characters. Chinese became established as the dominant cultural medium during the millennium (111 B.C. - 938 A.D.) when Vietnam was under direct Chinese rule.
Even after Vietnam gained its independence, Classical Chinese continued in use among the literati. In fact, the characters were in official use right up until the 20th century. It was only the abolition of the Chinese-style official exams in 1918, following on from China's own abolition of the exams, that finally sounded their death knell.
During the time it was the official language, Chinese had a massive influence on the Vietnamese language and literature. Even today, a huge proportion of the modern vocabulary has its origins in Chinese.
As in the case of Japanese, the original Chinese pronunciations were modified to suit local habits. In many cases the pronunciation of Chinese words appears closer to the dialects of southern China. Similarities can often be found with Cantonese, the language of nearby Guangdong province, rather than Mandarin. As a random example, the word 'superior, high-class' is gao1 ji2 in Mandarin and gou3 kap7 in Cantonese. In Vietnamese, the equivalent is . (It is interesting to speculate whether such similarities are due to closer contact with Guangdong than with the mainstream northern Chinese dialects, or whether Vietnamese simply retains older features of Chinese pronunciation, as do Korean and Japanese).
The story of Vietnamese writing does not end with Classical Chinese. Despite the overwhelming prestige of Chinese writing, the Vietnamese managed along the way to develop a system to write their own language.