Their are ways of treating vent gleet at home. Mainly washing their vent area, probiotics and ACV. But I had a hen a few years ago who got vent gleet after a hawk attack. None of the remedies worked and she died. I would try to get your hen to a vet who could prescribe medications or antibiotics to fight off the infection. Home remedies just aren't strong enough in my opinion to help them heal.
What do you suggest using to wash the vent? I have three that were in a dirty coop and we didn't notice it on them. But now they've been moved to a much better coop!