The Thai's migration into Vietnam was gradual, motivated by oppression on the part of the Chinese.  The largest Thai group, the Black Thai, had a semi-independent government near Dien Bien Phu in the early 1950's under the French.

One tradition holds that at the beginning of the second century AD the White Thai and then the Black Thai arrived to settle in their present area of habitation.   

Linguistic studies indicate that the Thai peoples first migrated to southeast China, where they developed their unique culture and language.  They then dissemminated throughout southeast Asia, and divided into several subgroups.  The conflicts between the Thai and the earlier Khmer peoples are well documented in Cambodia, but the situation is less clear in Vietnam.