Vinted complaint: Refund not received

Hello there, it has been weeks now that I am waiting for a refund. I have contacted vinted to let them know about it and it has been back and forth for weeks now. They asked me for my bank statement to proof that there’s no entry which I did. Now they are telling me that it isn’t enough they want a letter from bank to that no refund has been received on my account. I spoke with my bank who told me that they do not provide such letter for this matter. Very surprised vinted doesn’t understand that and still require a letter for me to get my refund back. I mean i cannot force the bank to go against their process and it sounds ridiculous from vinted i sent them an official document from my bank what else do they really need. I am now raising a dispute so I can hopefully get my refund back. In the meantime I want to raise a big complain about it please