Vibration probes are installed at a distance of the shaft equal to ca. - 10 Volts. Values in between -9 and -11 V are usually accepted as correct gap voltages. It is so because that is the center of the linear range of the sensor output (usually the linear range goes from 2 to 21 Volts or so). Note that this is true for 5/8mm tip diameter probes, other probes have different ranges.
Check the datasheets of the probes, you should see the output curve in there.