I have the smart case and I don't feel it on the glass. I barely even notice it now after owning it for around 9 months now. Tbh I would just get a refurbished Air 2 then wait for the Air 3 Imo. It should only be a minor spec upgrade and a 64GB refurb is a good price now
Turn your iPad Air 2 up to full volume and place your hand on the glass when playing a video and you'll clearly feel the vibrations, even with a smart case on. I'm not talking about with the smart cover on, as that defeats the object of the exercise if you're typing an e-mail whilst listening to music.
I did get a refurbished iPad Air 2 and now returned it because of this vibration problem. Fair enough if you're willing to put up with it or don't notice it anymore, but I'm willing to wait until October/November to see if iPad Air 3 will reduce this problem significantly. Another reason why I am not willing to put up with it is that there is no significant vibration problem with my iPhone 6 or iPad Mini 2, so don't see why there should be with the iPad Air 2.