If you already have VFD wired with relay turning spindle on/off I would recommend keeping that. The ModbusEZ plugin works the same as the HYplugin and also has a feature for Rpm only, this would just set your speed. IMO the plugins are much quicker in setting rpm and more accurate than using analog.
I have a manual 1/2 written for ModbusEZ plugin for a week now, if I stay not so busy at work I my complete today and post.
Dan