Son Tay was a smaller, leaner Op
I met an ex-Air America helo driver who'd disgree with that asessment. According to him, when the rescue was first proposed, it was to a really lean and mean op. using Air America pilots who knew the area and each other well.
He maintained that when the op.was first planned to go in, the PoWs were still there, but the delay in working up the more complex plan and bringing in the USAF aircraft and crews meant the PoWs had been moved before they went in.
<<Conspiracy theory alert!!!>> It might seem highly unlikely to some, but he also said he wouldn't have been at all surprised if someone on Kissinger's staff hadn't given the North Vietnamese a headsup to move the PoWs so the political loss of face wouldn't be too great, thus screwing up the peace talks.
Some reports say the prisoners were moved as little as 48 hours before the raid. If that's true, given the (hopefully high level of) pre-op scrutiny the camp would have been under immediately before the raid, many would find it hard to believe the Americans (or someone at the top) didn't know they'd been moved. Even if they did, an op. like that develops a momentum of its own, and the political pressures (possibly within the US military) may have been too intense to see it cancelled.