You usually need a spindle control board to translate the PWM signals coming down your parallel port (or in your case through the Smoothstepper) into a 0-10V or 0-5V control signal for the VFD, together with the stop-start signals.
See the PMDX 106 as an example.
There are some stepper driver systems that are all-in-one and include the spindle control e.g. the well respected Gecko G540.
Then in Mach3 the PWM signal is controlled on the SpindleSetup tab of the Port&Pins set up page. Also the SpindlePulleys set up.