If a vent prolapse in chickens is anything like a vent prolapse in ducks, you can try putting some hydrocortisone cream on it (anti-inflammatory)....with our hen who had a prolapse we also put some honey on it to reduce the swelling, and we held it in for long periods of time. You can try giving her an Epsom salt bath but since she is a duck that might not be the best idea due to the risk of her drinking a lot of the Epsom salt water. What helped with our chicken was we let her free range for a day and she got to walk around a lot which probably helped get the prolapse back in. But, that was with the help of 2 different types of pills that were prescribed to her by our vet