Vinegar is used to fix a dye. I Never heard that it would stop the action of bleach but I wouldn't personally rely on that method until I tried it on a small sample. I think the best sure way to stop bleach action is to wash it out. And maybe toss in a splash of vinegar with the soap. The Idea of using bleach to mottle fabric is brilliant however. Now why didn't I think of that? I'm seemingly always do that by accident it just seems to make sense to use it as a controlled method now that you mention it!