I've stayed in aft facing cabins a few times on Azamara and my answer on the vibration is it depends. If the ship is doing more than say 17 knots there is a lot of vibration, but then your itinerary may not call for that speed. However it wouldn't put me off and any vibration is going to be temporary if at all. Deck 6 has the deepest balconies of the aft facing cabins compared to deck 7 (less deep) and deck 8 (the least deep).
Phil