Excessive vinegar assaults the palate, invades the olfactory and even taunts the mucous membranes with its tingly temperament. You can't extract the vinegar taste from a dish, but you can cover, cloak and cache it by harmonizing taste sensations.
The success of the sugar-and-vinegar relationship hinges on balance. When paired thoughtfully in a gastrique or agrodolce -- the French and Italian sweet and sour reduction sauces -- vinegar and sugar create a combination that seems like one taste sensation; the line between sweet and sour blurs. You can take the concept of the gastrique and agrodolce and use it to offset an overly sour dish. If you want to soften the vinegary harshness of cold dressing or salad, mix in a pinch or two of powdered sugar. If you need to balance the acidity of a soup or sauce, caramelize granulated sugar in a saucepan until it darkens. Let the sugar cool for a few minutes, stirring it constantly, and add it 1/2 teaspoon at a time until the dish balances.
Mixing With Fats
You always need an acid to cut through fat in a dish. For example, lemon cuts the richness in hollandaise, and vinegar balances the creaminess of mayonnaise. And it works the other way around, but in different ways. Fat coats the palate and prevents some of the molecules responsible for sourness -- any molecule that drops a hydrogen ion -- from reaching the taste buds. Acid is also more soluble in fat than water. Unlike water-based ingredients -- stock, for example -- which release vinegar's aromatic compounds readily, fat tends to hold on to them, so less of them make it to the olfactory system. The less vinegar you smell, the less you taste. To alleviate some of the vinegary taste in clear soups, garnish each bowl with a generous drizzle of olive oil on top; to remedy purees and soups, stir in a couple of tablespoons of butter or heavy cream at the finish.
A Pinch of a Base
If you've mixed vinegar and baking soda to clean or to make a bottle rocket, you've seen the results of an alkaline and vinegar mixture. Numerous reactions occur when baking soda and vinegar mix. But the only one that matters -- when correcting for excessive sourness, at least -- is ion exchange. When baking soda takes a proton from vinegar, it turns it into water and carbon dioxide, making it less vinegary. If you add baking soda to a vinegar dish, you won't get an explosive reaction, but a cool mellowing of its harshness. Stir a pinch of baking soda into the dish and taste it. Add another pinch of baking soda if needed, but no more; too much baking soda creates an obnoxious alkaline taste.
The Hail Mary
When all else fails, make another batch of the dish without vinegar and combine the two. Nothing too scientific or complicated about getting rid of a vinegar taste by adding more of everything else, but it is a last recourse for severely soured dishes. Make the second batch and add it incrementally until the taste balances.