Culturally Vietnam is much closer to China than Japan cause it had been directly under the Chinese rule for 1000 years, almost every part of Chinese culture, festivals and holidays, literature, governance system, architecture, religion, taboos and superstitions...are exactly same building elements in the Vietnamese culture.
Vietnamese also have to study old Chinese history and literature when they are in primary schools, many Vietnamese can recite more Chinese Tang dynasty's poems than the Chinese, and when reciting those poems, they sound exactly like Chinese, we can fully understand.
Chinese language is very hard for westerners to learn, but for Vietnamese, most can speak fluent Chinese in 3 to 6 months learning.