The ACM seems to have been the source of the vibration. Either engine would effect the bleed air being processed. If the fuel cut off could have been inadvertently selected on one engine, coupled with a change in the vibration, flying very low level, vibration still exists, wrong engine? Restart? The whole scenario is very alarming, even when sitting in front of the TV at home let alone being in that situation. I am sure they done all they could as competant pilots, they were trying to return to land. There is no evidence that the engines would not have got them there, unfortunately they did not have the information available to us now, a couple of years after the fact. Also, I don't care about the circumstances of the flight, not my business, and I do not think it has any direct bearing on the cause. Just sad to say bye to more good people.