As I understand, integrating zxing or another method to read QR codes with Vuforia has always been a "third party" unsupported feature. Though - correct me if I'm wrong!
In the Unity manual, it specifically says Vuforia supports QR Codes??????????
https://docs.unity3d.com/Manual/vuforia-sdk-overview.html
Probably an error, should say something like "including QR like codes", referring to VuMarks.