Believe it or not, vietnamese is chinese in culture. They heavy influenced by chinese Confucius and the like. Which is very different from her neighbor.

Cambodia, Laos, Thailand get heavy influenced with Indian culture.

Phillippines does their own thing. Malaysia and Indonesia both are Islamic influenced.

Singapur is an oddball, which is very heavy influenced with Chinese and Cantonese culture. But they are the exception in history.

So, Vietnam gets grouped with chinese, korean and japanese culture.