Yup. It is important to keep vents free of gathered droppings. Lightly clipping feathers around the vent help with breeds like Wyandottes that have excess fluff back there. Like Sourland mentioned, it sounds like maggots. Droppings sticking to the vent area stay moist, flies lay eggs, maggots crawl around, and if there's a wound from where the bird picks at the vent, the maggots have a burrow. Prozap Screwworm aerosol on the wound wipes them out. Maggots fall right out when hit with that stuff. Clean the area after like Sourland said with a rump bath. A shower head works well for that. If maggots get in the vent, you may need to give the bird an enema. Dry the area and use a triple antibiotic ointment on any wounds and lightly spray the feathers around the vent with Martin's Horse and Stable spray, Adam's fly repellent or something similar. That will keep the area from getting re-infested as it heals. Check it daily and clean the area with a moistened soft cloth or paper towel. Oxine AH is great for cleaning wounds. So is chlorhexidine or even Betadine.
