No, the servo won't run with a VFD - it might turn, but there are only a couple of VFD's on the market advertised as being able to run permanent magnet motors. My Hitachi WJ200 claims to do it, but the best one appears to be the Invertek series. And those are easily as expensive as a dedicated servo amplifier from the motor suppliers.

My advice, for what it's worth, is to replace the bearings in your spindle now - maybe $500 for some decent but not champagne-level AC bearings. Then keep running your machine while you plan out a spindle upgrade. Sounds like your spindle isn't going to last long enough to get parts together for an upgrade.

In addition, you can make an aluminum cooling ring for the spindle nose and run flood coolant through it. I have a 3D model of one I made and used until I needed the clearance back.

Tripletree CNC is the same outfit I tried to contact - thier website is upgraded + new business name, but same address. Maybe they've changed since I tried to engage them.