Your nickel (which is really 25% nickel and 75% copper) reacted with the vinegar (acetic acid) to form copper and nickel acetate. You could use the solution to electroplate stuff.
I think vinegar (or any other weak acid) preferentially dissolves the nickel. Nickel salts tend to be lighter greenish-blue, copper darker blue.
Know what the MSDS says about chemicals you play with. I suspect the quantities mentioned are smaller in effect, but someone may decide to completely dissolve coins to completion. NiAc has the following in its MSDS: Copper Acetate:
Good grief, never mind. Detecto made it sound fun until electrocution, coma, diahrea and death. Sent from my SGH-T499 using Tapatalk 2