i do something similar to that charcoal method, but with graphite, for a more grey color.
there are art pencils that are all graphite (no wood), and are GREAT for that.
scrape it with a razor, then rub it in.
Just the same for a flak vest, scrape and rub in. It should work just fine with your vest, unless it's something like nylon, which might have a hard time taking the weathering.
That graphite pencil idea is GREAT! I have a ton of those (im an artist) I'll use charcoal and graphite then. *bows* You guys/gals rock!
I know what you guys mean about being nervous to weather your stuff. I have a pair of Chang boots that I got on ebay that I spent waaaaayyy too much for a few years ago that are still in perfect, out of the box clean condition. I just have a hard time bringing myself to "ruin" $300+ boots! That's why I'm saving that part for last.