Older MP electric distributor

Shane,
The vacuum advance should only be operating at light load, small throttle opening cruise. Are you getting detonation when you stomp on it or as you roll into the throttle? If it's happening when you stomp on it it is likely the advance mechanism is hanging up and not releasing the advance fast enough. If it happens when you roll into to it adjusting is what it needs.

I use a mighty-vac hand held vacuum pump to check and adjust the vacuum advance. If you pump the mighty-vac to get full advance and release the vacuum the advance should go back smoothly and quickly, if it doesn't look for something hanging it up and fix as needed. You say it's an old MP unit, does it have the adjustable mechanical advance or is it pre 2000 vintage that is just a stock distributor with light springs? If it's a stock advance mechanism they are known for hanging up.

The other thing you should do is check with the mighty vac is what the vacuum is to start the advance moving and what it takes to get it to full advance. To adjust it put a vacuum gaue on your engine and see what the vacuum is when cruising at a steady speed on a level road. Then adjust the advance using the mighty-vac to be full in at that point. If this causes detonation when rolling into the throttle you can then back it off until it stops.

If you don't have a mighty-vac it is a very useful tool. Besides using to tune your advance it comes with bits and pieces so you can use it to bleed brakes without an extra leg to pump brakes. The all plastic ones are in the $20-$30 range and the all metal ones are up in the $60 range.