Sadly you did damage the shells by the use of vinegar, which is an acid.
Acids etch the surface of the shell.  In fact, given a long enough exposure
and/or a strong enough acid, they will completely dissolve the shell.
Proper use of bleach would have solved your problem without damaging the
shell, but you are beyond that point now.  Rubbing in mineral oil or baby
oil, then wiping off the excess, will probably improve the appearance of the
shell, making them look more or less as they look when wet, though the
original brilliant gloss may never be completely restored.  Some people
prefer to use WD-40, or a type of furniture polish call [something]gold, I
don't remember the exact name.  Don't use vegetable products like vegetable
oil or olive oil.  These will produce the same effect initially, but may
later turn rancid and discolored.  Good luck.

Paul M.

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