I've been following this thread for a week now since I also noticed this issue and figured I'd chime in. I haven't gone through all 30 pages, but is there anyone that doesn't have this vibration issue? Or found a display model that doesn't?
I think I've only come across two users in the Apple forums who claimed they don't have this issue:
https://discussions.apple.com/thread/6629452?start=0&tstart=0. I find that a bit surprising (unbelievable?) since it certainly seems to be the norm that the iPad Air 2 will vibrate based on the comments here and elsewhere.
I'm not sure how I feel about this issue yet, but it definitely felt a bit awkward playing Monument Valley with a vibrating screen due to the low/bassy tones that game has. I'll most likely be keeping mine since I got a great deal on it and everything else about it is perfect. Hopefully I'll be able to get used to it that I don't even think about it anymore.