Even among the many AFL nationalities, the source of the Vietnamese workers is difficult to explain. How and why would the IJA have imported workers from Indo-China. The short answer is that they did not. They were already present and were simply ‘drafted’ into service. This also partially explains their rather small estimated number of only 200.
As was explained elsewhere, the monks at Wat ThaWorn Wararam follow the Chinese sect of Buddhist that is also prevalent in Vietnam. Locally the slang name for this temple is Wat Yuan. Yuan being the Thai term used to describe people of Vietnamese ethnicity. It is said that there had been a community of Vietnamese in the area dating back to the mid-1800s. Exactly how they came to settle here is shrouded in the mysteries of history.
So it is highly likely that there were a moderate number of young men of Indo-Chinese ethnicity when the IJA staff arrived to establish their HQ at ThaMaKam. They would simply have been impressed into service on the TBR. Exactly where they worked did not survive the post-war purge of records. In fact, the only tangible evidence of their existence is the plaque dedicated to them at the Thai-anusorn memorial.