In the 1980s, how widespread was the belief in the US that there were still American prisoners of war in Vietnam?

Being Italian, I've always considered Rambo II "just" an excellent adventure film, not giving much thought to the plot point about American POWs. But lately, I'd remembered that there had been other similar films (rescue missions for American prisoners in Vietnam), so I wondered if American audiences simply considered it a pretext for making good war films, or if anyone actually believed it.

Which, when you think about it, doesn't make much sense. If there were really American POWs, why keep them in the middle of villages and not in a city prison? If the Russians were really involved, why not take them directly to the Soviet Union? Or simply kill them outright? Because surely, if it had been discovered that there were still any alive, a president like Reagan would have reduced Vietnam to the Stone Age.

Thanks in advance to anyone who can satisfy my historical-sociological curiosity 😊