The dyeing isn't ever exact, cause it has to do with the amount of tannins in the leather, and the concentration of the vinegarroon. I always use distilled white vinegar, but that part probably doesn't change anything. You can always soak the leather in it longer. You can also do multiple dips in the vinegarroon. If you think its not dark enough, dip it again. Another ting worth experimenting with is brewing some black tea, and applying the tea, and letting it soak in before dipping in the vinegaroon. Tea has tannins, so that can help intensify the effect. I've never needed to the that on leather, but I use that when aging wood with vinegarroon. I've noticed leather usually looks a little grey until I oil it with neatsfoot. Once I oil the leather, it significantly darkens, and mine is usually winds up very deep black with no grey left. Make sure after dipping it in the vinegarroon to also run it through water and baking soda to neutralize the acid. Also I normally let it dry about 8-12 hours or so, and then I oil it. Once I didn't oil it for a several days, and the leather turned hard and brittle.