Buy a book of find an eBook on tanning leather its not rocket science veg tanning is thousands of years old. Tannins are used for vegetable tanning that's why they are in the hides and is the main ingredient in the process they literally soak the hides for months in the solution( made from woods high in tannins ,Oak for one,. how much is left in the leather depends on the strength they used and how much they rinsed out after the hides were done, if I remember right its the ph they worry most about. But all veg tanned hides have tannins. if you sealed the leather then used tea the black probably will rub off or bleed. The iron in the vinegaroon reacts to tannin you can put tea on wood and it too will turn black in fact it is used as a stain for wood as well as leather. The roon can be put directly on oak for example as it has mucho tannins.