I think that's the acid in the vinegar working at the leather: you need to cancel it out with a base.I have done the vinegaroon dye, steel wool soaked in vinegar, and find that it rots the leather even if it is promptly oiled after the drying process. Anyone else have that issue?
You can (somewhat) ameliorate the dry/rot issue by giving the piece a good dousing in a thick baking soda solution prior to wax/oil/dubbining the piece.
I've not had the problem in my later pieces since I started using that process - but even the oldest of those pieces is only a couple years old, so I can't say from personal experience if the lifespan's as good as leather that's never seen the stuff.
SPEAKING OF - I just used the purchased bottle of walnut dye for more than a dab of spot treatment, and I can now DEFINITELY say there's some vinegaroon in there.
Over lunch break I gave some some pieces of cut/punched goatskin a quick soak in it, patted them dry, and sat them out in the sun.
Two hours later I looked in on 'em - with my regular walnut they'd be just beginning to brown.
These are almost black - one piece that characteristic blue-black of vinegaroon.
I brought them in, scrubbed them hard with aforementioned making soda, rinsed them out a few times, and now have them on the counter trying.
Maybe they'll end up a slightly deeper brown than I was going for: maybe I just lost a goatskin and a day's work of layout and cutting. Guess I'll find out in a few hours.
:/
(Not saying this stuff still doesn't have a place in my toolbox - just that I'm gonna reserve it for the hard-to-dye cases like commercial goatskin)