This paper observed the phenomena of language contact in progress by investigating the pronunciations of mandarin of Vietnamese immigrants. During the research, the author collected recording data of 16 hours long, including word lists, paragraphs, and conversations. Listening experiments were also conducted. All these data were analyzed in order to describe the language behavior of the informants under different situations. Sound systems were also established based on the relationships of phoneme and sound variations. It was found that different types of data have different characteristics which reflected different perspectives of the Mandarin sound systems of the informants. Under this multi-dimensional observation, this paper has described the Mandarin sound systems of Vietnamese speakers and also demonstrated the variations under different situations.