I'm not sure how well a piezo beeper will respond to vibration. Something like a piezofilm accelerometer may work better, depending on the frequency range you are interested in. These sensors will be sensitive in the direction perpendicular to the plane of the piezo beam, and insensitive to vibration in the plane of the beam. Three sensors arranged triaxially will give you a axis, but it won't tell you the direction of the source along that axis.
If you want to pinpoint the source, you'll need multiple accelerometers and a spectrum analyzer with cross-correlation features.
Are you able to initiate the vibration on purpose for whatever medium you're trying to detect vibration from? I ask because you must take adjustment into account if you want to set the sensibility once for all..