VPEX, with its controlling bridge (CB) and bridge port extenders (BPEs), is analogous to a chassis with slots.
When you enter VPEX mode (see enable vpex), the CLI prompt changes to show that the CB is in VPEX mode and is slot 1 in this context:
Slot-1 VPEX X670G2-48x-4q.14 #
In VPEX mode, you refer to ports in the familiar slot:port notation in applicable commands, where the CB is always slot 1, and the BPEs are slots 33–64 (see Extended Bridge—Chassis Analogy).