Looks like Mac Virtual Display uses Sidecar tech from the iPad. You can’t connect to both the Vision Pro and iPhone Mirroring at the same time.

The image shows an alert message on a screen, with a background of tall trees in a forest. The message reads: “Unable to Connect to iPhone Mirroring. Jarrod’s MacBook Air is currently using Sidecar.” Below the message is a “Try Again” button, and there is an icon of a smartphone above the text.