I haven't tried the vinegaroon method with wood (I don't think), but used it almost exclusively with veg tan leather when I was making straps and belts. Even though the black doesn't get quite as deep as with dyes, it definitely produced a more natural black that didn't have the characteristic blue, purple, or weird green cast that some inks/dyes do when they fade. Also, vinegarooned black never faded on my works because it was a chemical reaction not a pigment. The level of tannins in a piece is also influenced by natural processes, so it is not guaranteed to be even throughout the piece. I had some leathers that would be harder if not impossible to get as dark as others, and sometimes there would be unevenly dark areas (I'd hesitate to call them splotchy though). Would anticipate the same in wood.