Stophel and Mark, I have not had that type of experience with Tandy or with the use of Vinegaroon.
FWIW, just about any application of any dye to veg tanned leather will result in the leather becoming stiffer. It will also happen if the leather is wetted such as in preparing a bag to be turned. The leather needs to be returned to it's pliable state with neats foot oil or some other leather dressing.
When you make your vinegaroon you need to use enough iron to totally neutralize the acid in the vinegar. It takes a while to make it and even after a long period of time it may not be "done" if enough iron has not been added to the venegar to complete the reaction. When all the metal has been devoured it may not be ready. The way to be sure is to add more metal and also check it occasionaly to see if there is pressure in the container. Use the plastic container that you bought the vinegar in and cap it after adding the metal. Everything you read on the interweb will tell you not to close it tight but if you close it tight and check it about once a week by loosening the cap you will know when it is ready. If you open it and there is an escape of gas from the container, it is not done making itself. If there is no escaping gas it may be done but if the metal is all gone you may need to add more metal and check for gas in another week.
If you are in a hurry, Fiebings UMC Black will give you a fine black color.
I found out by experience about what I explained above. I used a quart bottle of white vinegar to make a batch by putting some 0000 steel wool in the bottle. When all the steel wool was eaten I tried it on a test piece of leather. Got a gray blue color.
I poured some shavings from a brake rotor drum turning machine into it and left it for a couple more weeks. Checking it during that time there was a pressure build up so I knew it was working. At the end of just over two weeks it stopped building pressure and there was still metal residue in the bottom of the bottle.
At that point I used it to dye the bag below. The bag os not braggin work, just plain a bag to serve in everyday use.