I wish this were true but the avilable evidence does not support this belief. If the North Vietnamese did withold American POWs in 1973, then they are long dead. Consider that while it is possible that the North Vietnamese could have witheld Americans as a possible "bargaining chip" in the event that the U.S. violated the terms of the Paris Peace Treaty (the U.S. did not, the North Vietnamese did), what could they possibly have done with them after 1975? They could not very well announce to the world what they had done. Even Communists propagandists would have been hard pressed to explain the sudden appearance of six hundred-plus Americans who they had repeatedly denied existed.
If the North Vietmamese and Pathet Lao forces did withold Americans, they have long since starved them to death or murdered them outright in order to cover up their deceit. It defies belief that the North Vietnamese would risk international condemnation (as weak and ambivalent as that would have been) by keeping these people alive. Since they were listed in most cases by U.S. Forces as "Missing, presumed killed", all the North Vietnamese had to do was simply remain silent and murder them. Then they were "killed in action".
It is far more likely that the overwhelming majority of the Americans listed as MIA today were intentionally murdered by North Vietnamese and Pathet Lao forces soon after their capture. After Action Reports of SOG teams who made insertions into Laos indicate that Americans who were taken prisoner were usually gratuitously tortured and then murdered soon after their capture by local commanders.
Many who were captured alive were subsequently murdered by VC and NVA forces in the process of being marched to POW camps in Laos and North Vietnam. A good example is the case of an American missionary working at the Leprosarium in Ban Me Thout. Betty Olson was murdered by her North Vietnamese captors after she became ill from Dengue Fever. She received no medical treatment from her captors, and when she became too weak to walk they shot her. This was apparently routine treatment by people who Jane Fonda and Tom Hayden refer to as "heroic freedom fighters".
There is also a certain percentage of Americans downed in Laos who probably successfully evaded capture only to die of starvation or exposure in the wild jungles of eastern Laos. If you have never been to the mountainous Central Highlands area of South Vietnam, which extend westward into Laos, you have no appreciation as to just how wild and trackless the area is. It can, and did, hid divisions of troops. It is not hard to imagine downed pilots and SOG personnel wandering around in this wilderness unseen by friend and foe alike.
What is a sham is the North Vietnamese effort to help locate and return the remains of Americans listed as MIA. Totalitarian governments such as North Vietnam's record everything in minute detail. I believe that they have recorded the location of every single American aircraft lost over land in Southeast Asia during the war. I also believe that they have detailed records as to the fates of the crews and passengers aboard these aircraft.