I was reading a manuscript today called Random Accounts from a Mountain Residence (Sơn cư tạp thuật). It is supposed to date from the late 18th century, but I found one account in it which appears to be from the mid nineteenth century. This account is about brothels. The passage begins with information from the Chinese classics about the establishment of brothels in antiquity, and then it talks about Vietnam.
It says that “formerly” there were a lot of prostitutes in the areas of the capital where troops were stationed. Many of the prostitutes had venereal diseases. This illness was colloquially called the “Siamese sore” (暹羅瘡, Xiêm La sang) and was supposedly contracted by soldiers who campaigned against Siam. After they returned they infected the prostitutes who then infected others, and so on and so forth.
The text then says that after the 戌甲 year, whoever was stationed at the capital got this disease. It could not be cured, and some people died from it, and efforts to prevent its spread did not work either. Eventually the prostitutes were found, their heads were shaved, and they were chased away.
The 戌甲 year could be 1728, 1788, or 1848. I’m guessing that this text is talking about 1848, because the main wars with Siam occurred in the 1830s. In fact, however, I don’t know how likely it is that Vietnamese soldiers actually had sexual relations with Siamese women during these wars. Most of the fighting between Siam and Vietnam took place in Cambodia and southern Vietnam. So it’s not clear to me if the “Siamese sore” really was from Siam.
Nonetheless, what I found interesting is that I actually found a premodern source which talks about this.