Why am I unable to use a prepaid Vanilla gift card with my Apple ID?

Are you entering them as a payment method in your Apple ID Account (not your Wallet app)?

Add a payment method to your Apple ID - Apple Support

It may be more nuanced such as your card not having something else Apple wants such as an actual billing address. What exactly is the error message?

You say it worked in the past but not now (which kind of goes against everything I have seen about Vanilla cards to date). Remember, nobody here works for Apple so some questions you may have to refer to Apple to get any kind of authoritative assistance. Even then I have found you may have to find somebody who really knows Apple's detailed policies about something. This support article has various country telephone numbers for contacting Apple for support and service ➔ Contact Apple for support and service - Apple Support

You may wish to check what Vanilla says:

https://www.vanillagift.com/help

Apple's policies may vary from country to country. This is an international forum and we have no idea where you are. Even then, you would have to find somebody here who had experience with your country. What you read in other posts may apply to those countries but not yours.

Just an FYI, Vanilla Visa cards are issued by 5 different banks and lately many(all?) Vanilla Visa cards have been invalid for the Wallet app and use with Apple Pay. Those already in the Wallet still seem to work. I don’t know if all issuing banks or just some may have dropped support.

By "other posts" I meant in other threads over the years.