As someone who knew guys on the POW/MIA Task Force in Vietnam from the mid-90s onward, I can tell you that nearly all the surviving POWs were executed around 1986. The Hanoi regime can't admit it without losing face and thus the bodies haven't been returned to the families.
By 1986, the Hanoi regime was losing financial support from the USSR and needed IMF funds to stay in power. They knew that if living POWs were found in violation of the Geneva Accords, it would make it hard for them to get IMF funds. Thus they were killed.
The Task Force members sometimes spoke of a couple MIAs they found who had integrated into small mountain villages and for whatever reason have been allowed to live with their new families.
Two or three MIAs may still be alive today, but not 250.