Turntables thump when turning off?

"The thump is loud, very loud if it's at the full volume"

You know what the Doctor says to patient?

It hurts when I do "x" Doctor-then don't do "x"

Contact VPI.  Some of their turntables had an issue with making a pop when powered off.  They sent me a capacitor for free for my Classic 2 and it was a pretty easy fix and resolved the issue.

Ground the motor.

Try the above suggestions, you can do it.

Hard to imagine that a snubber cap across the power switch would be missing or broken. But I would definitely check.

More distance between turntable power switch and phono pre might already be the solution. Is the power cable to the turntable very close/parallel to the interconnects from the cartridge to the phone-pre? After all the snubber only minimizes the problem and doesn't neutralize it totally.

If nothing works and you are really “desperate” you can build in an extra switch that only turns off the low voltage connection to the motor. You would lose the warranty but the problem would be fixed. But I would first try disconnecting the motor manually (unplug the cable if possible) instead of using the switch and see if the problem is gone.