Evidence points to 1,000-year-old relics in today’s Sayabouly Province as proof of early settlements established by migrants from Burma and China. A wave of Tai Lue arrived from China in the 15th century, before the Lane Xang Kingdom claimed Sayabouly 100 years later.
It briefly fell under Siamese control, due to its vulnerability as the only Lao province on the Thai side of the Mekong. But in 1904, Siam was forced to cede the area to French colonials. Thailand annexed Sayabouly in 1941, naming it Lan Chang Province, but it was returned to Laos after World War 2 in 1946.
Most Hmong villages in Sayabouly did not see any fighting during the Indochinese Wars, and Hmong leader Vang Pao did not recruit any of his men from the area. After Lao achieved independence, Sayabouly became a fully-fledged Lao Province.
Sources: Walker, Andrew (1999). The legend of the golden boat: regulation, trade and traders in the borderlands of Laos, Thailand, China, and Burma.
Fadiman, Anne (1977). The Spirit Catches You and You Fall Down: A Hmong Child, Her American Doctors, and the Collision of Two Cultures.