I have never heard the term Circular vibration and I possess 36 years of helicopter maintenance and teaching.. All helicopter vibrate..a lot... they inherently vibrate due to the constant leading, lagging and the flap and droop of the main rotor blades as well as inclement weather etc..Helicopter manufacturers realize this and incorporate several devices to counteract this vibration.
One has to realize that a rotor head behaves somewhat like a gyroscope... and like anything that spins fast like for example your car tires thay need to be balanced with weights. A component of flight control rigging is rotor track and balance. Weights are used to balance the rotor head as well as adjustments to get all of the blade to track together.