The piece thaat makes my vitamix turn fell off

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.

I have a large set of slip joint pliers that I place jaws up in the vice and engage into that large nut up the base of the jar. Here is a link to a photo on an auction site local to me to show what the pliers I use look like, it's the New Zealand eBay equivalent site.
http://www.trademe.co.nz/building-renovation/tools/hand-tools/pliers/auction-817772271.htm

Sounds like it may need a new blade/bearing assembly or coupling or both.

you can check this out, maybe what you're looking for?

Amazon com vitamix wrench assembly Home Kitchen

If you Vitamix is new you should be returning it for under guarantee inspection, there is something not right.

If it was me I would make one.
Find some kind of rubber material that would work and use either rubber cement or super glue.
If you can't find the material around your house I'm sure you can pick something up in the hardware section of any large store.

I use my imagination a lot as you can see.

Vitamix has a 5-year warranty. Go to their website and they will send you an empty box with a UPS pick up tag and fix it.

We just had this problem.  Called VitaMix and they talked about resetting the thermal protection switch inside.  They suggested lifting the base down firmly, but gently, on the counter to help the internal switch reset.  I did it while they were on the phone and, sure enough, the machine came back to life.  Hope this helps.

first check the blades on the container thay should turn easy by hand if not thats your problem , you will need blades . blades turn easy ? you will need to replace the motor . the temp /overload has changed and is tripping to easy . this is common with people that use there vitamix for tough chores a lot . a lesser blender would just burn up a motor in a puff of smoke .a vitamix dies this way . in the end its the same you will need a motor

My Vitamix 3600 plus has same problem. Result of detachment of rubber coupling between lower splined socket from blade shaft. This is a tough rubber molding that takes tremendous punishment through years of smoothies. Mine showed some rust on the bottom of the blade shaft, where it parted. I tried gluing back together with super glue and tried to eyeball it. Of course it was off by a few degrees. When I reassembled and turned it on, what a racket of vibration! But it held. Amazing. Obviously not good to go though. So I took assembly all apart and redid it, chucking the shaft and the lower nut in the lathe. This time, all true on axis. Glue held for about a week. Right now I am regluing, this time with epoxy. We'll see how it holds up. And meanwhilst, I'm researching replacement parts! I'm not too optimistic about the epoxy.