Hello Louis
When we talk about “what VLAN CDP will use” this only has to do with trunk ports. Any access ports used to interconnect two switches will still exchange CDP packets regardless of what VLAN those ports are assigned to. In those cases, CDP packets will be exchanged without VLAN tags, just like all traffic in and out of an access port.
For trunk ports, CDP will always use VLAN1 on switches since VLAN1 can never be removed from the database. If the native VLAN remains VLAN1 on a trunk port, then CDP packets will be sent untagged. If the native VLAN has been changed on a trunk port, CDP packets will be sent tagged on VLAN1, even if this VLAN is not allowed on the trunk!
I hope this has been helpful!
Laz