Vinegar will strip bluing but I don't think it affects nickel plating.
To strip nickel at home, Brownell's sell a chemical, but it's expensive.
A chemical is also sold by Caswell........
Plating Kits - Metal Strippers - Caswell Inc
The best way is to have a plating service electro-strip it for you.
When stripping plating you have to get ALL of it, or any tiny areas left will cause a chemical finish like bluing or Parkerizing to appear to have white freckles.