Unfollowing is not reciprocal. You can unfollow a user while they continue to follow you. To remove them from your followers, you must block them (which will also unfollow them).
Ensure you are logged into your account. You cannot unfollow anyone as a guest. How to Unfollow Someone on Vinted
When you unfollow someone on Vinted, the following changes occur: Unfollowing is not reciprocal