I have done some vibratin sensors using a old headphone speaker with a small ball point pen spring superglued to the cone , I have tried perpendicular and parallel, #2 works better, but I had to use a LM386 to amplify it and used a germanium diode to rectify it. it might be possible to use a opto isolator in place of a amplifier, just depends how much voltage you can get out of a vibrating speaker, usually not much.