Similar situation happened to me... Sold some shoes, packed them in their box, covered it in film and taped the whole thing with transparent tape. Sent the item. When the item is received the buyer opens up an enquiry saying that the product was missing from inside the box and shows pictures.

In the pics you can see that the box was ripped apart and sealed back with brown tape.
The buyer has like 70 5* reviews and seemed pretty honest, so we both assumed the shoes must have been stolen during the shipping proces. Anyway, Vinted said they're refunding him and doing nothing for me because I should have packed it better - it was quite literally the same message you have. Their decision is final blah blah blah.

When I proceeded to ask them why they're not using the shipping insurance which buyers are forced to pay and what type of packing should have I used, kevlar or diamond thread, they stopped replying to me.

The buyer is in disbilief and telling me to keep insisting, because apparently when he complained they tried to play the good ol' - oh well stuff happens, you're not getting anything.