what is social distance? Discuss the social distance between Japanese and Chinese; Germans and Vietnames ?

I don't know what subject you're studying here, or what type of social distance you're asking about. I'm pasting a link below. If it looks similar to what you're studying let me know.

Use of the concept of social distance dates back to Georg Simmel's discussion of the stranger in his Soziologie (1923). According to Simmel, the stranger represents the union of newness and remoteness as he moves out of one social circle and strives for acceptance in another. Robert Park (1924) popularized the concept of social distance as the grades and degrees of understanding and intimacy that characterize personal and social relations. Social distance is based on social norms that differentiate individuals and groups on the basis of race/ethnicity, age, sex, social class, religion, and nationality. The greater the social distance between individuals and groups, the less they influence each other. It was Emory Bogardus (1925) who operationalized and measured social distance by first asking over 200 participants their willingness to admit members of 39 different racial and ethnic groups to the following: close kinship by marriage, as fellow club members, as neighbors, as workers in their same occupation, to citizenship in their country, as visitors only to their country, and as persons to be excluded from their country. The Bogardus Social Distance Scale is largely synonymous with the concept today.