Hubby and I just finished doing holiday baking all day and we had a recipe for Penut brittle which has vinegar in the recipe.
What does vinegar do in candy making??
Vinegar lowers the pH of the candy (i.e., makes it more acidic), so that when one adds the baking soda (basic) carbon dioxide is released from the reaction between the two.
These carbon dioxide bubbles are forced through the liquid sugar that's been cooked to hard-crack stage. Makes the candy more "foamy," so when it cools, the candy is lighter.
I need to look this up in one of my favorite sites, cooking for engineers, to confirm that I've described this correctly.
Here is a decent explanation: