My peafowl only get the head lice, which can be *really* hard to see, and you're right, you won't see them on the vent. When I spray I spray the vent, legs under the wing, and I very carefully try to get some on the head and neck. Permethrin, when used as directed has a zero day egg withdrawal.
I found a a link with some really good pictures of lice and mites.
http://cpif.org/wp-content/uploads/2016/03/AmyMurillo.BackyardFlockPestsandManagementTechniques_Oakland.pdf
Poultry head lice (Cuclotogaster heterographa):
Red mite (Dermanyssus gallinae)
Bed bugs...
Figure 1...