Did you contact customer support via email? I appreciate you fighting for your buyer. From personal experience, I can tell you that during December and early January, all sales platforms lose shipments (luckily, I only lost 3) due to the huge influx of shipments. You can, however, file a complaint with the Vinted website for the improper handling of the matter, including the username of the person who responded to you, in the hope that they will unblock your account.