Power feed on table usually is run with 120V, separate from the spindle motor. I haven't run a mill using a VFD yet, but I have looked into it. Advantage is that the speed can be controlled instantly without changing pulley size. Some info here --> http://www.practicalmachinist.com/vb/transformers-phase-converters-vfd/bridgeport-vfd-ridgid-tapping-278423/
I run my lathe and Bridgeport using a rotary converter. I bought the control box from Anderson Converters http://andersonconverters.com/page2.html 3HP panel is $169, then just add a 3HP 3phase motor as an idler. Very simple to wire, and works good. I just bought a 10HP panel from them to set up another converter for a larger lathe and surface grinder.