The San Chay joined forces in the 1860's with a powerful Hmong army in northern Vietnam.  They took large tracts of land and raided Buddhist temples of their gold.  The San Chay were induced to follow a self-proclaimed Hmong king named Sioung.

        The San Chay believe that they originated in the area where China's Hunan, Guangxi and Guangdong provinces meet.  They migrated to the Yunnan province of China, and those who live in Vietnam continued south in two main waves.  The first migration epoch happened in the early 17th century, and the second in the early 1800's.  Some believe that they first settled in the Bac Bo upland area.

        Linguistic evidence provides some clues to this movement of people.  Some scholars believe that the group known today as Cao Lan stopped for a considerable time in Tai-speaking areas of south China, and largely adopted a Tai language similar to Nung.  The San Chay group, on the other hand, moved through Chinese areas in Guangxi and adopted a form of Cantonese as their tongue.  It is theorized by some that they originally spoke neither a Thai-Tai  nor a Chinese language, but rather a Yao dialect.