Lets imagine that I have big LAN environment with many switches and many VLANs, for example, VLANs 5,15 and 50. I have PVST+ activated, and all of my switches have same (STP) priority.
I connect a new switch, with factory default settings, to my environment. And let's look only at VLAN 15. Nothing will change in this moment. Now, I create VLAN 15 on my new switch. (I didn't create a VLAN 15 SVI or add any ports to VLAN 15 - I only created VLAN 15.)
Which MAC address of this new switch will be used for the Bridge ID of VLAN 15? Will that Bridge ID be used in the Root Bridge election for VLAN 15?