The Ngai speak a Sino-Tibetan language.  Specifically, some (Dan subgroup) speak a dialect of  Cantonese.  Their language is a completely different language group from Vietnamese, which is of the Mon-Khmer group.  

It is clear that all Ngai speak a Chinese dialect.  It is not clear that the dialect is Cantonese in all cases.  There are five sub-dialects of Cantonese, which does confuse the issue.

     Ropal code for Cantonese (YUE) (YUH00)