The vax cards are HHS documents, so technically forging them is a federal crime.
So probably something you can get away with if you just want to get into the local steakhouse, but probably not a good idea to try at airport security.
Yes, exactly. Very stupid to try to forge it for Federal purposes. I note this morning that the iOS 15.1 update now has a COVID-19 vaccine passport built in, and you can upload the QR code from your vaccination record to import the record into the mobile wallet.