I just went through this earlier this Spring. Prep H doesn't work. The internal tissue that is exposed is too slippery. I ended up not using any of the advice I found on the net since none of it was working. Went to Tractor Supply and bought a can of Blue Kote. It makes a protective covering on the vent that is literally blue/purple. I also used vetericyn wound & infection care. Its pricey but good stuff and can be used on us too for various wounds. Treats a lot of different things. Usually I used warm baby wipes to clean her vent area. I held them in there to let any nasty stuff soften so it could be wiped off. Let it air dry for a moment, then I sprayed the vetericyn on her and let that dry. Followed up with blue kote. I also clipped the bum feathers that were in the way. She was dripping non-stop of urine so she had quite a mess. It took a while for this to heal up but without any real manipulation, it healed up on its own. I just cleaned it as needed and did the wound care described above.
This happened to a chicken of mine too. Keep her separate from the rest. Keep her in a somewhat dark room. If she gets to much light she will want to lay another egg and that won't be good. A had my girl in a small dog crate and during the day put a light blanket over the cage. Keep up with the Preparation H. It does work, just have to do it almost everyday. Mine was in the basement for about 2-3 weeks. When she was healed fully we let her back out with the rest if the girls and she is good now. Took her a few days to lay again but she is still good.
I had a chicken that had a prolapsed vent and somebody told me to put some granulated sugar on the tissue because it shrinks the tissue. It really worked well for her and it doesn't hurt her either! You will want to keep a close eye to make sure it doesn't happen again, but you will have to repeat this, or any other treatment for a few days at least. Luckily for me (or rather, my chicken), it cleared up the first day and she hasn't had trouble since then. Hope this helps out too.