How do a clean port stain from the inside of a crystal decanter? I
have tried using white wine vinegar to no effect

Dear mattcallaghan-ga,
 
My wife uses a sterilising tablet which you would use for baby's
bottles. Fill the decanter with water, drop the tablet in and leave it
overnight. The stain will comes off on its own, or a quick rinse with
warm water will do it.

This forum discussed the same problem and soda crystals were
suggested. The asker found that they solved the problem
http://forums.moneysavingexpert.com/showthread.html?t=15919

A similar discussion on this site came to this conclusion.
?I've always used 1/4 cup of rice, and then added a water/ vinegar
mixture. The rice acts like little scrubbers.?
http://www.foodbanter.com/wine/23264-decanter-cleaning.html

Hope his solves your problem.

answerfinder-ga