What has worked very well for me in the past in similar situations (not with kitchen appliances but any older item needing a part) is find people selling the item or parts for the item on eBay, and then messaging them asking if they have a source for whatever it is I am looking for.
Almost always this leads to what I need, whether it is for a camera or an old watch or a vintage computer or whatever.
Messaging through eBay is easy, fast, and free.
Simply type Vitamix 4000 in the Search box, and get the listings displayed, and then click on Contact Seller.
eBay will send the message, and notify you when there is a reply.
As the override gizmo will not reset when cooled down you do know that it is faulty so replacing it is going to have to happen anyway if you are repairing it. It may be just that item that's gone bad or it may have been made bad by the motor drawing excessive current. The proper way to check current draw would be by running it through an ammeter. The last thing you want is a faulty motor, they cost a fortune.
You will need to source them from a bearing supply company as Vitamix only supply the bearings as a complete unit with the blade assembled. Once you have the bearings out take the sample with you to the bearing supply company for correct identification.
Put your meter across the 2 disconnected potentiometer wires and turn the knob and see if you get the sort of variation you expect to see. The pots do give a little bit of trouble and aren't worth very much, I'd be inclined to throw one in anyway, the very last thing you want it to be is the PCB, they are not cheap.