Give the explanation about what kind of English that Vietnamese users of Engish learn areSociolinguistics
Sociolinguistics focuses on the effect of a given society on the language. It also studies how language differs between groups within a culture which are separated by variables such as gender, religion, ethnic background, etc. Children growing up in a given society learn to distinguish the sounds of the language from birth. Second language learners, however, do not learn the sounds this way and must learn them through language study - listening, speaking, studying grammar and vocabulary, etc. With regard to some Asian language speakers, including Vietnamese, the sounds of English are very difficult to assimilate. In Vietnamese, a different pronounciation or different syllabic emphasis can change the meaning of a word. A native speaker learns these linguistic attributes as a child growing up in the culture. A native speaker of Vietnamese, therefore, does not have English sociolinguistics.
Sociolinguistics also can refer to levels of language. For example, the use of slang would not be appropriate (socially) in a formal setting, business setting or educational setting.
I am not sure from your question to which aspect of sociolinguistics you are referring and, if it's the latter, perhaps the question is asking whether Vietnamese users of English are being taught substandard English. Some language programs have been criticized for teaching substandard English to non-speakers, not just Vietnamese English language learners.