From In Buddha’s Company: Thai Soldiers in the Vietnam War, by Richard A. Ruth (U. Hawaii Press, 2011), pp. 168-169:
Thai language skills seem to have spread quickly to areas beyond the villages directly surrounding Bearcat Camp. Infantrymen on operations were surprised to find Vietnamese women in isolated villages who could speak some Thai. Yutthasak Monithet, who went to Vietnam with the Black Panther Division’s third phase in July 1970, recalled conducting impromptu Thai lessons for curious Vietnamese: “As for the Bien Hoa market. people in the shops could speak Thai, but they spoke it as if they had [recently] learned Thai. Sometimes they had questions [about Thai], and they would ask, ‘What is this thing called in Thai?’ We would tell them the words that Thai people used for these things.” The market that Yutthasak described is fifteen miles or so from Bearcat Camp.
The other factor that contributed to the spread of Thai was the influence of ethnic Vietnamese who had lived in Thailand and Laos. There is strong anecdotal evidence to suggest that some of the Vietnamese refugees who had lived in Thailand in the 1940s, 1950s, and 1960s returned to Long Thanh District and settled in areas near Bearcat Camp; others found their way to Saigon, Vung Tau, and other R & R towns frequented by Thai troops. Some of the repatriated Vietnamese opened Thai restaurants while others provided Mekhong whiskey and other goods to sell to the Thai soldiers. Many spoke the Isan-Lao dialect, “as they do in Ubon [Ratchathani] and Nong Khai, and others spoke Central Thai, also known as Standard Thai.
A third factor was the role of the Thai-Vietnamese translators. Some of the Vietnamese who were hired to translate for the Thai units had lived in Bangkok before the war. Unlike the Vietnamese who settled in Isan, these Vietnamese learned Central Thai, the country’s official dialect. They lacked Thai citizenship and apparently had been repatriated along with the Vietnamese from the northeast. Their familiarity with Vietnamese and Standard Thai made them a valuable asset to the Royal Thai Army and the Royal Thai Navy as they sought translators for their units.
Repatriated Vietnamese were mediators between the Thai military and the indigenous communities. The Thai volunteers relied on them for items that the US Army would not or could not provide. In market towns such as Long Thanh and Bien Hoa, Viet Kieu (expatriate Vietnamese) restaurants were centers of Thai relaxation and recreation. Chanrit Hemathulin’s unit regularly patronized one of these restaurants near Bearcat Camp because it offered northeastern Thai staples, such as lap (minced-meat salad), som tam, and khao meo (glutinous rice). “It was as if they were Thai restaurants, he recalled….
Mixed in among the population of Vietnamese returnees were Thai women who had married Vietnamese men back in Thailand and then accompanied them to Vietnam when the Thai government had deported them. Like the returnees among whom they lived, these women served as mediators between the two cultures.
The Chinese characters for Viet Kieu must be 越僑: 越 as in 越南 Yuènán ‘Vietnam’; 僑 as in 华侨/華僑 Huáqiáo ‘Chinese Abroad‘.