“Where would one start now?”
By you & everyone contacting your congressmen/women, senators & POTUS and DEMANDING a full accounting.
The Vietnamese Commies are enjoying trade relations with us so there is your carrot.
Take away the carrot and see how fast the Commies come up with an accounting.
One of the finest men I have ever known is Harley Hall. He was the last Naval Aviator shot down in Viet Nam, on 1-27-1973, the day the Paris Peace Accords were signed. His back seater was released during Operation Homecomming but not Harley. He was left behind. Read the very sad truth: