Some pictures would be helpful, but in my experience a dirty butt is usually just from a heavy worm load, bad food, or an unbalanced diet. With vent gleet there is often a white, smelly discharge from the vent and some white crust around the vent. If they butts are really dirty I'd give the soak/clean them up and dry them off, then you can keep a better eye on what's going on.
In general treats shouldn't make up more than about 10% of their diet. Some of my girls binge on fresh grass in the spring and that can cause some messy bottoms. It might also be time to give the feeders and waterers a good clean if you haven't recently. I do use acv water but not all the time... It should only be kept in plastic waterers, and while it probably isn't causing the problem you could switch to plain old fresh water and see if there is any difference.
Worms is another common cause of messy butts and DE really isn't going to do anything (it's also dangerous if inhaled so I don't use it). If they've never been wormed and you can;t find any other reason for the dirty butts I'd deworm everybody with either safeguard or valbazen.
