One cup vinegar to 2 cups water
ETA. I followed these instructions
How to Dye Your Nylon 3D Prints<br/>
Was pretty much following your lead on this, and I largely followed those instructions, as well. As said previously, I'd had the polished white parts from Sculpteo soaking in tap water (with just a touch of dish detergent) for the past couple days. Shell, button, cap, and ring were thoroughly sanded on their exteriors; no sanding on the internal. I decided to just dye the whole shebang with the black Rit powder and call it a day.
I filled an old pot (ca. 6" diam. x 3" deep) with enough tap water to submerge the parts and then (based on your success) added probably a quarter cup of vinegar. Brought that to a simmering boil on the stove, transferred the parts from my soaking can, and let them bubble for about thirty minutes. Then added the entire package of Rit powder and boiled for another thirty while constantly stirring/agitating with some impromptu tongs bent from a clothes hanger. Removed the pot from the heat and allowed to soak for another hour with a paper towel on top of the parts to hold them down. Took out the parts and rinsed thoroughly.
Here is the result with a few quick coats of Krylon #1311 matte to seal after thorough drying. I stuck my black wrinkle Reo Mini 2.1 in for comparison and forced the iPad to overexpose slightly so the color can be seen. It's more a slate black, however the color was deeper and closer to the Reo when wet, and I suspect that would remain the case if a gloss sealer were used...
I had been curious as to whether the sanding would affect the dye absorption, but the color seems to have taken equally on the internal...
There is some slight color variation in the grooves that doesn't show in the photo. The one place where the color did tinge toward blue as others have experienced was at the bottoms of the recesses for the 510 and atty o-ring. I had shaved these down with a chisel X-acto blade to compensate for surface sanding the top...
I'll likely add a few more coats of Krylon with wet sanding between to even out the remaining variations caused by the uneven pores in the plastic. I still might try my fret wire ring idea, as well. Also waiting on replacements of my broken button magnets to complete assembly.