I refer to the vinegar/steel wool mixture as woodturner's douche(WTD).
I've used walnut and oak... Both worked well. I just threw a wad of steel wool into about 6 ounces of vinegar and left it for a week or so until it dissolved. I've read recommendation to strain it through a coffee filter to remove the steel particles... Otherwise they might rust at some point down the road. I've never strained mine.
The tea trick I've read about but never tried.
I'm not sure the vacuum chamber will work, but I'm interested to see your results if you try it. I may be wrong, but I think it requires oxygen to work... I'm probably way off base on that point.
Sharpies seem to have a kinda of bluish tint that I don't like. Ink works, but the stuff I've tried also has a bluish cast. I've gone to using the alcohol based dye from CSUSA when I'm trying to ebonize something. Although I've never used the black version, I've used RIT dye for coloring stuff previously... The knock against RIT is that it's not colorfast. I've seen shoe polish used, but I've never done it myself.