I remember when the Vietnam POW's came home on stretchers and in hospital beds. They had been starved, tortured, and their injuries and illnesses had gone untreated. I don't remember McCain specifically, but it was clear these people had been through hell. Many did not survive.
In the mid 1980's, we started taking in large numbers of Vietnamese refugees from the camps - the boat people. San Jose was one of the biggest resettlement destinations. Years later, I attended a function also attended by a retired South Vietnamese army general, living in the local retirement community.
There was some bitterness about the resettlement. Apparently, former members of the Viet Cong and possibly some North Vietnamese military types, including possibly some from the POW camps, may have slipped in as well.
We have been effective at ferreting out and expelling Nazi concentration camp guards that sneaked in as refugees after WWII. I don't recall any mention of expelling former POW camp guards from Vietnam.
For a guy that graduated fifth from the bottom of his class at the Naval Academy and supposedly got in more trouble than almost anyone else, he had a remarkable career. He was a steadying force in Congress, which is something we could use more of now.