I'm not a holster maker at all, I make motorcycle seats. I've done this vinegaroon several times with great success and never had the seat change color when the leather flexes.
When I vinegaroon a seat, it only takes about 10 minutes to apply the vinegaroon. I wipe my mixture on the leather with an old shop rag. It starts turning black instantly. I wait about 7 - 10 minutes for the vinegaroon to soak into the leather completely. Then I rinse the leather with clean water to wash away any residual vinegaroon. Once it has been rinsed, I soak the leather in a heavy mixture of baking soda and water to neutralize the vinegar. This is an important step because the vinegaroon will continue to wage war against the leather causing it to weaken, get brittle and eventually, fail if the vinegar isn't neutralized. Once I have soaked the leather in the Baking Soda solution for about an hour, I pull it out, blot it dry and set aside to dry overnight.
Overnight, the leather will dry to a blue-gray hazey color. It won't look very appealing yet, but you're almost there. I always take the leather and roll it around to examine the piece for missed areas or other things I may have overlooked. Once I'm satisfied with the outcome, I grab the Neatsfoot Oil and start applying in small Wax-on, Wax-off motions and really work the oil down into the leather. I apply a coat, then wait a bit for it to soak in, then apply another coat. I end up applying several light coats of Neatsfoot Oil to the leather and the Indigo Black color starts to appear. Once I'm happy with the color, I apply a coat or two of Satin Sheen to add a little luster and send it down the road.
Hope this helps....