You don't want to just rinse the vent. That is not going to help much at all other than irritating it. She needs to be in a tub of warm water, down in the water, with her vent submerged. Put some epsom salts and sugar in the water. If you don't have epson salts just use sugar, or just the warm water. Let her soak in it for at least 10-15 minutes. Do this several times a day. This will help the prolapse naturally shrink and go in on its own. Use lube like KY jelly (without additives) or some vasoline or olive oil and gently push it back in.
i'm not sure about the nutritional value of bread and milk, or how that helps. You do want to make sure she eats something else, like her regular chicken feed. Chopped hard boiled eggs and yogurt would be good.