I had a different issue with Vestiaire (slashed leather, the seller hid the cut in the pictures) and thankfully recorded the unboxing on video too. After two unsuccessful emails to them I filed a chargeback with my bank (paid with Cc) incl. all evidence (video, pics, correspondence, return shipment slip). My bank refunded me the money the next day. Funnily enough Vestiaire came around on the same day and agreed to do a refund after all… I told my CC provider and in the end I received the refund from VC too, while the CC refund was reversed.
Once you have written communication from Dior that they refuse to acknowledge your claim I would simply file, not waste any more second trying to speak to their customer service. It’s unacceptable what they are doing to you.