vinegar in lollipops
From what I could fine in a cursory search of recipes, the typical ratio for vinegar candy is 4:1, sugar to vinegar, usually done as 2 cups sugar, 1/2 cup vinegar.  

The highest ratio I found was 3 cups sugar to 1 1/2 cups vinegar (Dottie's Vinegar Candy Recipe).  That, however, was specifically for a soft candy--taffy.  

I'm not sure whether the higher ratio means that that mixture could not reach hard ball stage if heated high enough or if it's merely the choice of the recipe writer to make a softer candy.