Yes, many illegal immigrants in Vietnam when I was there. Mostly anti- Castro Hispanics. But also anti-Communist Eastern Europeans and Asians.
Back then the background checks were a total joke. I know. I saw many of them, including my own, which was a joke.
Personally, I was more worried about some of the natural born citizens who would do anything for booze.
background checks are still a joke and besides the Chinese and whomever else have all of them.