These sign languages may be related to the native sign languages of Laos and Thailand. French Sign Language appears to have had an influence on Vietnamese sign languages.
Woodward, James (2000). Sign languages and sign language families in Thailand and Viet Nam, in Emmorey, Karen, and Harlan Lane, eds., The signs of language revisited : an anthology to honor Ursula Bellugi and Edward Klima. Mahwah, N.J.: Lawrence Erlbaum, p. 23-47
Woodward, James; Thi Hoa, Nguyen; Tran Thuy Tien, Nguyen (2004). Providing higher educational opportunities in Deaf adults in Viet Nam through Vietnamese sign languages: 2000-2003. In: Deaf Worlds 20: 3 (2004) - pp. 232–263