Were those advisors Russians or Chinese?
The white advisors seen and possibly overheard on radio communications were tentatively identified as Russians but my friend Mike Benge, AID provincial advisor in the Central Highlands SVN, wrote a lot about Cuban interrogators in NVN.
See if he wrote an article in the Vietnam Veterans for Factual History “Yellow Book” series on this. I know he had a number of articles on this subject published in newspapers and they should be on the internet. VVFH.org. As I mentioned in a reply elsewhere, I haven’t unpacked all my VN/Cambodia files so what Mike wrote is probably still in one of about 15 boxes in my office/storage room.
However, in Cambodia, high ranking government military officials showed me a diary, in Chinese, showing the travels of a Red Chinese soldier from NVN to Cambodia (assuming that this is an authentic document). They also showed me a brilliant red Mao collar button, about 1-1.25 inches in diameter found on his body.
I asked the Cambodians about his body but they said it was too damaged and they buried it. I wanted to exhume it in order to check the cheek-bones and teeth (aka shovel shaped incisors) which are different from those of the Mon-Khmer linguist/physical grouping.
I have a BA in Anthropology and field “digs” experience with eastern U.S. American Indians. Teeth were always a marker for many racial groupings. It would have been at least informative to have seen the deceased teeth.
My Cambodian hosts offered several possible origins of this communist soldier, including Red Chinese advisor, Red Chinese combat soldier, Chinese person born in Cambodia and sent to NVN for training before returning to his birth-land to operate among leftist Chinese citizens, etc.
In fact So. vietnamese, So.Korean ROK in VN and Cambodian troops, and possibly US troops reported intercepting enemy radio communications in these languages. I don’t know know w of any specific article or publication on this but I assume that they exist, especially in MACV provincial reports and CIA/DIA intercepts/reports.
Hope this provides you with a little insight into the complexity of this war.