I still have a couple of birds who are showing this problem. What I am doing now is spraying the vent area with Blu-Kote as it is gentian violet which is the best thing in the world to heal yeast infections. This will keep the skin protected from the burning of the yeast.Update. Almost 3 weeks later and no real improvement. She's isolated from the other chickens. We've been bathing her daily, giving yoghurt in feed, spraying iodine on vent after bathing and adding vitamins and probiotics to her feed too. Took to vet 5 days ago and he prescribed antibiotics (terramycin) for 5 days as was sure the diarrahoea was caused by a parasite infection.
She's eating and drinking well and looks otherwise healthy with glossy feathers but can't seem to shift the crusty vent problem. Only real improvement is that the top of her vent is now a rosy pink rather than a disgusting yellow mess, the bottom half of her vent, however, is blocked with the yellow "scrambled egg" crust and she still smells really bad!
Am going to give it another week, but it's starting to look like a hopeless case!