In his Việt Nam: A History from Earliest Times to the Present, Ben Kiernan argues that in the early history of Vietnam there were two important migrations of peoples into the Red River Delta.
“By the time of the classical Chinese contact with northern Việt Nam, the early ethnolinguistic pattern there had been transformed by two external influences from the south and north, from mainland Southeast Asia and southeast China.
There are people in Khammouane Province in Laos who speak a language known as Saek (Sek). In the twentieth century, Western scholars struggled to identify what language family this language belongs to. The earliest scholars claimed that it was Mon-Khmer, but eventually French linguist André-Georges Haudricourt made a convincing case that it was a Tai language, and more specifically, a Northern Tai language.
Linguists believe that Tai languages emerged in the area of what is today Guangxi Province in China. Out of some proto-Tai language that existed some 2,000 years ago emerged Central Tai, Northern Tai and Southwestern Tai. Of these three, Southwestern Tai is the one that emerged the latest. Linguists now say that it emerged around the eighth or ninth centuries CE, and that its speakers started to migrate away from the “Tai homeland” at that time as well.
As the map below indicates, these three branches can be identified with different areas, and the place where the Saek language is spoken is in an area where one would expect to find Southwestern Tai speakers, not Northern Tai speakers.