I would use any antifungal cream (check it has an ingredient that ends in 'azole' - like miconazole and does NOT have pain killers included as they can be toxic to chickens).
I would definitely apply it to the area around her vent. But some sources I have read say you should also give it orally as the yeast infection can be systemic not just local.
Using antifungal cream orally is also what you do for sour crop so it isn't quite as weird as it sounds!
But I am really, really not an expert -
@casportpony is someone I trust on dosing things like this.