Implications of different spindle configurations

So - I am running a retrofitted (3 axis + spindle) machine using repurposed ultra3000 drives. I want to be able to do rigid tapping, and a variety of other jobs that require precise control of the spindle. Right now, I'm using a 0-10V analog signal to control spindle speed, but this doesn't seem to work very well for position. What I need is a better understanding of the choices I have available. The issues that I am encountering include vibration (the PID loop uses the spindle encoder to attempt to regulate position using voltage), and poor orientation performance. I know how to work the orient module, and I've already got a nice g-code script with a prologue and epilogue written in python to do the heavy lifting. I am willing to rip it all out if necessary.

Can I use 0-10V for precise control? I've seen and heard of others succeeding.

How do I reduce vibration in the system? I can tune PID parameters and FF gains, but I need a very tight loop to make sure that my accuracy is sufficient. This seems to lead to more vibration.

Downstream, the vibration is producing very unreliable velocity measurements from the encoder. I don't want to put a lowpass on, because that will just slow down the feedback.

Any thoughts or solutions would be very welcome.