I wasn't suggesting you buy that motor, it was just an example of a "real" motor with a "real" encoder mounted.
Sensorless Vector drives I have used in the past specify speed regulation to +/- 1% or less. That means that the actual speed is within 1% of the setpoint. That means that your "S-word" is the same as the motor speed so you don't need an encoder. You know it is correct so there is no need to measure it.
You can setup the spindle control to output a signal that can be converted into an analog voltage to control the VFD.
Matt