The San Diu speak a form of Cantonese, or Yue, one of the major dialects of China.  Cantonese is a Sino-Tibetan language, very commonly spoken in Guandong province of China, which is said to be the place of origin of the San Diu people.  Their language is a completely different language group from Vietnamese, which is of the Mon-Khmer group.  

Some sources have reported that the San Diu actually speak another dialect from Guandong called Iu Mien.  But, though some San Diu may speak Iu Mien,  most probably speak Yue.  

     Yue (Cantonese)  is a Chinese language, a branch of the Sino-Tibetan family.  Ropal code for the Yue language: YUH00