Is it possible, or just urban legend?
One wonders what the advantage would be to Vietnam to still hold POWs, especially secretly. The last thing the country needed, especially in the 1970s, were more mouths to feed. This much has never been explained to me.
There are tons of western tourists these days in SE Asia (including large numbers of Vietnam vets) & one’s movement is not really at all restricted, or monitored in any meaningful way. If anyone wanted to investigate POW camps at this point, it wouldn’t be that incredibly difficult. I should think we’ve have had credible sightings at this point. There have been a couple of outright hoaxes/scams involving “photos of POWs” from some unscrupulous types looking for cash, but nothing concrete.
Vietnam has even been pretty cooperative of late in returning remains of KIA, even as the country copes with 300,000 MIA of their own.