What component is a VZ reference designator for
VZ is sometimes used as the designator name for zener diodes.  VZ1 and VZ3 look like 2-lead packages, so that fits.

Take one off the board carefully.  Put a 10 k&Omega; resistor in series with it, then connect that combination to a power supply, like maybe 24 V.  Measure the voltage across the device, flip it around, then measure the voltage again.  If it is a zener diode, then it will be 700 mV or so in one direction, and some volts in the other.  That latter is the zener voltage.