rotor Dynamics

What do you want to measure:

Velocity, displacement, acceleration and do you want to measure these relative to the bearing housing or relative to the surroundings etc etc.

Many hand held instruments from the likes of SKF, B&K, etc that use accelerometers or you may want a non-contacting system such as Bently to measure actual shaft movement relative to the bearing housing.

Mold soft wax in box around shaft halfway between bearings in two sections and run the machine. Remove the wax, measure it.

If the shaft isn't running very fast, a dial indicator may be used to get a fair indication of shaft runout. Note that you will have to sorta guess at the limits of pointer vibration.

If you're interested in finding the load on bearings, mount an accelerometer on a centrally located bearing and measure the acceleration, do a little math, and the amplitude may be squeezed out of that, but this method is least accurate for amplitude, however it does yield information about load on bearings.