How to measure the frequency of vibration of an ERM motor?
One revolution of the eccentric-weighted shaft should correspond to one cycle of vibration. The motor shaft speed should be directly proportional to the motor voltage. With a simple armature voltage control, using the armature voltage will not be and exact indication, but it might be close enough depending on the reason for determining the frequency. However, direct measurement of the frequency and displacement of the item under test may be required for a meaningful test.

If you have a motor with an eccentric weighted-shaft that is designed for the purpose of producing vibration, there would be no reason to look for something else unless there is some aspect of the operation that has not been described in the question.