While I was away the replacement part I ordered for my VP-X turned up, so tonight I spent an hour or so carrying out the necessary repairs.
The VP-X power stud is simply a standard Eaton part that is cut down, so, with the broken part as my template, it was a simple task to modify it. I then reassembled the VP-X and reinstalled it onto the sub-panel.
This time I actually connected the power cable before bolting the VP-X in place. This way I could get a torque wrench onto it, just to make sure I didn't again snap the power stud off. 35 in-lb is not very tight, but I am assured it is more than sufficient.
With all systems plugged back in, and wiring resecured, I then powered my panel back up and carried out some tests. All good.
The replacement part on the left, the broken part on the right
Using a torque wrench this time when doing up the power cable connection
