You can typically find replacement carbon brushes for Vitamix blenders at appliance repair shops, online retailers, or directly from Vitamix. Ensure you get the correct model-specific brushes for the VM0145 blender with a 3Hp motor. Double-check the compatibility before purchasing to ensure a proper fit.
Check the black driving clutch on the top of the motor that engages with the other clutch half on the base of the blender jug. The fault should be there, normally it will be the motor part.
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.
Some models had a weakness with the variable resistance in the rotary switch but your description sounds more like the main PCB or even the motor both of which are very expensive parts to replace.
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.
The wrench hole is hidden firstly by the large rubber part that holds the beaker and secondly by a blob of clear silicone that covers the hole for your allen key
No because there is very little metal in contact with the food other than the stainless steel blade and the stainless fitting below it neither of which should be leaching anything into the food.
Have a close look at the blade and the fitting under it for any signs of corrosion.
You should write (not an email) to the Company about this issue and maybe they have some explanation because these are expensive machines.
