Try some ACV in it's water ... 1T to a gallon of water, ACV should be the undiluted with the "mother". Use only plastic containers as vinegar & metal don't like each other. Check the temp in the brooder. If it's drinking alot of water, poop will be runny.
Not sure what you're meaning "protruding"... Looks ok to me but very messy. Clean it off (warm water soak IF the poop gets stuck/crudy), maybe it's my eyes but vent looks ok. It's possible it looks like it's protruding due to the "diarrhea".
Do you have other chicks that hatched/purchased/same age as this one?
What are you feeding?