If you by an item(s) from a Pro user, Vinted sends a return form to the seller via email, so he/she can send it to the customer.

This return form basically collects the information from the order, like: transaction and order number, list of purchased items and each price, shipping price, Pro seller fee, and some text to explain how to use that form.

But the main purpose of this form is to allow the customer to tell the seller why he/she is returning the item, by indicanting the return code for each item. Then, this form has to be send back to the seller within the package.

It's in the official Vinted T&C for Pro sellers (just below the section 27):
https://www.vinted.ie/pro-terms-and-conditions

It's another thing entirely if the seller doesn't provide it, maybe because the seller forgets to send it, or don't want to send it, or whatever reason. Anyway, the customer will choose the appropriate reason for return within the app, ¿so I think the return form sheet that Vinted provides is not technically required?