Why not blame the preamp? Why not blame the AC cords or the ICs to the phono stage? If the stock cap works for 80% of the VPI tables, I would look long and hard at other components' placement and emi/rf sensitivity as well. Some components do not play well in proximity to some others. I have to be careful with placement of my phono stage and its separate PS to avoid problems like low-level AM radio and various hums reproduced through the speakers. Careful placement away from my amps and especially, the VPI SDS gets me superb sound while sloppy placement gets me mysteries to solve. Some placements I tried while getting to the current one also caused the turn-on thumping, but only when using the switch on the 2-motor/flywheel body rather than the SDS. This does not happen in my current configuration using all the same components and cables.
Solving the problem at the TT end might keep you from finding a solution that is also causing you not-yet-detected problems elsewhere in your system.
Solving the problem at the TT end might keep you from finding a solution that is also causing you not-yet-detected problems elsewhere in your system.