Why does the VPI motor thump

The thump is the collapse of the magnetic field. Some caps work better than others at killing it (basically slowing the collapse ofthe field down with the cap in there.)
So even the same TT with the same supposed cap one might not thump and another one will.
Cap have BIG variances. the 'good' one may be well over rating by 15%. the poor ones well under by 15%
so that is a 30% variance.

VPI folks may be annoyed at being asked to fix it. But they should do more to stop it.
It is a FAULT, and not a normal TT feature..

I've had a few good turntables. My Thorens never thumped. My Rega always thumped, and my Linn never thumps.

In my opinion, it's a flaw in turntable design. It's easy enough to implement a properly sized capacitor across the on/off switch so that this is never a problem. As a TT Manufacurer, VPI is aware of the thump potential and should design their tables to never thump.

Just another reason why to me VPI always see,s like a work in progress.

Before flipping the side of an LP I always turned volume down to 0, flip side, drop needle, turn volume up. I have never experienced any thumps with my Regas - P5, P7, and current P9.

Try plugging in the power cord on the VPI into one of those power strips with an on and off switch and turn the power on with the switch. If the thump goes away then you will know it is a bad switch on the VPI! I replaced the switch on mine with a heavy duty toggle and never had the problem again. It's worth a try.

Is there a material that can shield from the magnetic field?
I would buy another capacitor from radio shack if I knew what type and size to get. Does anyone have any info on that?