Are you sure a fox bit her? She didn't have a prolapse before injury? Looks like she had a prolapse that went untreated and now the tissue exposed looks black,which could mean necrosis has set in(tissue is dead)this is very dangerous as infection will occur.Usually you need to remove the necrotic tissue by debridement,but b/c you are not sure what is exposed i would not suggest it. Is a vet an option? If not all i can suggest is clean exposed tissue apply an antibiotic ointment(nothing ending in "cane/caine"toxic to chickens) to area and see if tissue heals(not hopeful,but worth a try)can you see how far damaged tissue is in her vent? The exposed part could be a prolapse of part of the intestines or uterus. In treating a normal prolapse you would gently push prolapse back in and apply preparation H(shrinks tissue)others on forum have used honey or sugar,and hope that prolapse stays in place. I would start her on antibiotics for infection ASAP.