I'm hazarding a guess here, but it appears a combination of factors lead to this accident.
The guy who got shot came up behind the group just as a covey of quail was flushed, and was in tall grass, which may have partially obscured the vest.
If Mr. Cheney turned and focused on a bird, objects behind the bird would tend to disappear, because the eye is focused on the bird.
So, what I'm getting from all this is a confluence of events that ended in this mistake.
Mr. Cheney's friend being a few seconds earlier or later is probably the difference between getting shot and not getting shot.
My point is that we weren't there and it's easy to sit in judgment from a distance.
It is still the responsibility of the shooter to make sure no one is downfield. And from what I have read, the entire hunting party was acting in a disorganized manner. I would have expected better from an experience hunter like Cheney.
Bingo! When pheasant hunting with a friend, the dog flushed the bird, I focussed and sighted the bird, it flew over my friend, and I swung my gun high and did not fire. I talked to Ron later about this, and he said that this is not an uncommon situation. He's been shot twice in SD. He's an AVID bird hunter.