With MX's it gets a little funky admittedly; vap0 is indeed normally what MR's use for their first SSID, but for wireless MX's and Z1s and Z3s, the first SSID is actually vap1.
So what is vap0 used for on MX? Without getting too deep into the weeds, it's reserved specifically as a mechanism to apply things like content filtering, L7 firewall, and other settings to devices and VLANs that don't otherwise already have a group policy applied to them, which is why you sometimes see it referred to as the MX reserved SSID in errors.