I've sold stuff on Amazon. They only mix inventory for third party sellers, and then only if the sellers opt in to mixing inventory. If the seller is listed as Amazon.com it definitely came from Vitamix.

The key is to look at the "Sold by" field. If it says Amazon it's fine. If it says a well known restaurant supplier like KaTom or WebstaurantStore AND the "Ships from" field also says the same, it's also OEM because authorized Vitamix suppliers aren't going to risk selling off-brand parts.

Here's some examples.

Copycat:

https://www.amazon.com/Blade-Replacement-Vitamix-Container-VM001152/dp/B0B38WKZKP

Real Vitamix from restaurant supplier:

https://www.amazon.com/Vitamix-1152-Wet-Blade-Assembly/dp/B0028Y4CT6

Real Vitamix from Amazon:

https://www.amazon.com/Vitamix-15990-Vita-Mix-015990-Assembly/dp/B00EMQY19Y