Do you have any Amoxicillin, Tetracycline(s) or Baytril?
Metronidazole would not be my choice to treat infection like this, but do your research and you may find it would work o.k. It's most commonly used to treat Canker (Trichomoniasis) in poultry.
I have no idea what the cream is.
You can use raw organic honey if you wish or the mineral/coconut oils.
The main goal is to keep the tissue moist with ointment. Not sure I'd use vaseline gauze, it will get poop on it and you would have to change frequently.
Infection is possible, you may...
I agree, it looks like she may have prolapsed at some point and the tissue has dried, become necrotic.
The material that's surrounding the vent will become hard and crusty, so soakings and keeping that moist can be helpful.
If you don't have the honey, I would use whatever you have on hand...