Do I need a VFD?

I am using the stock Grizzly G0704 spindle and I have a MESA 7i76 + 6i25. Do I need a VFD to connect the stock Grizzly spindle to the MESA board?

No.
If you read the manual for the 7i76 it will talk about analogue spindle control. Then the signals can be connected to the stock control board.

I've actually just replaced my spindle with a DMM AC servo setup. Talk about a complete reduction in noise.

That's good news, one less item to purchase. Do you mind me asking which DMM AC servo you purchased? Maybe also if there is a guide to making a setup like yours as well that you can link to me?

I went with DYN4 drive and a 750 watt servo motor.
No guide.
Quick run down.
1: Buy servo drive and servo motor.
2: Buy Mini Pro belt drive conversion kit.
3: Machine replacement motor mount to suit servo drive.
4: Whilst head is off do 3 bolt mod, google for this mod.
5: Buy filters, AC line reactor and contactor as per user guide.
6: Do some research and decide how to have Linuxcnc control drive
7: Do more research
8: Walk towards a corner, assume feral position and spend a week or so comatose realising how much you spent, as an option to feral position, just sit down and rock backwards and forwards.
9: Wire everything up, optionally decide to completely redo control cabinet as everything else is apart anyways.
10: Configure Linuxcnc to suit.
11: Have fun and ignore how much you have spent.

Unfortunately there is no published article that holds your hand. It’s part research and part putting on your big boy pants and doing some thinking of your own.