There was a popular idea for a long time, which persists even today, that after the Vietnam war was over, Communist Vietnam still held a significant number of American POWs which it refused to release, and that were swept under the rug by the US government because it did not want to deal with their recovery. You can see this idea in a number of films from the 80s when it seems the idea was at its peak. This was all very conspiratorial and investigations never supported the idea, and eventually it faded. Another problem with the idea is that the Vietnamese really were not vindictive in their nature about the war once it was over.
The challenge, therefore, is to find a way to have Vietnam keep American POWs after the war.