The Yao have been classed as speaking four main languages:  Mien, Miao(a Hmong Dialect), Zhuang/Dong (T'ai/Kaddai Family), and Mandarin Chinese.  But the "true" Yao are probably those speaking the Mien language.  The others may be other ethnic groups that adopted the Yao name as a refuge from taxation or persecution.

The Mien language is a Miao/Yao branch of the Sino-Tibetan family.  It has been divided into four dialects:   1.  The Mien, with five variants; 2.  The Mun;  3.  The Byau Min;  and  4.  The Yau Min.  About 85% of the Yao speak either Mien or Mun. 

This means that the Yao language is linguistically unrelated to Vietnamese, the national language of the country.