Strongbow, that is an awesome bow, I like everything about it, the tiller is perfect on it. Nice work on the details, too.
I use pretty much the same stain on a lot of my bows. A couple things I found out seemed to help.
Before I put the steel wool in the water, I rinse it in a lot of hot soapy water. Sometimes there's a little something on the wool that prevents it from reacting with the vinegar. What you're creating by doing this is iron acetate.
If you want an even darker, richer black, brew some extra strong black tea, and let it steep for 24 hours. Paint that on to your wood, let it dry, and then apply your iron acetate.
The color does indeed react with the tannin in the wood, and red oak, which I love, has quite a bit. Painting on tea puts more tannin in the wood for the iron acetate to react with.
Here's a pic of the 3rd bow I made, a red oak pyramid bow. This bow sucked, but the stain came out nice. It's coated with some clear poly. Ignore the icky spot where I took off the handle wraps and never replaced them: