Hi,
Do you mean you want to reuse qrcode? that's not a good idea, qrcode should be computer generated and always be unique if your intend is single use. I read from your post that you use vuforia, which leads me to believe you are simply using vuforia image target recognition, which is not like reading a qrcode, if so, this is not ideal, and you should use a real qrcode reader, and generate unique qrcode for each needs, and associate that qrcode with the actual person account id from your app.
If you want to make a fidelity card, you need a user management system to begin with, do you have one. Can user log in to your system? when you have that, you'll be able to create a unique qrcode that contains the account Id it's associated with, and you prevent cheating and reuse of the fidelity card all together that way.
When you scan the qrcode, you check the content and if the Id matches the user login account id, then you can use it, else, you simply say, it's not valid.
This way, you don't need any server to handle usage of the qrcode.
https://assetstore.unity.com/search?q=qrcode&k=qrcodeBye,
Jean