How do you minimize stepper motor vibration?
What you need is sinusoidal current drive.

In other words, you have to treat the motor like a traditional brushless motor, rather then a stepper. This requires pretty specialized stepper drivers, and is not simple.

A simpler alternative might be to try microstepping the stepper motor, but that won't get you perfectly smooth rotation.

Really, for situations where you need extremely smooth rotary motion, a stepper is really just the wrong control system. You should use a brushless AC motor, or at least a brushed DC servomotor.

Here is a decent white-paper on stepper drive modalities, with some contrasting to AC synchronous motors.