Never tried vinegar.....so I'm kind of guessing here.
Vinegar probably etched off the polish/out layer on the buffalo horn ferrule. You can try mineral oil, or a furniture polish wax from the hardware store first to see if it will bring back the shine.
If not, you're going to have to repolish the ferrule with something abrasive. A buffing wheel attachment to a drill like:https://www.google.com/shopping/product ... IwBEPMCMAk
Should do the trick.
If you can get to the rust spots, remove them with some high grit sand paper.....try 600 grit if you can find it. If that doesn't remove it, go down in grit (coarser) until it does.
Before you etch the blade, you should seal the gap between ferrule and tang if you plan to dip the blade that far.