Under conditions of high manifold vacuum, which would add advance, you are still not building
peak cylinder pressures, nowhere near what you would have at WOT.
I don't see how vacuum advance could contribute to pinging unless something besides manifold vacuum was activating it.
I’ve had a few engines that experienced pinging at part throttle. Easiest check in the world to simply disconnect and drive it. I had to dial back my Mallory made Mopar adjustable vacuum can on my current 440 to avoid it, that was in conjunction with an all in at 2300 rpm mechanical.
