Vinegar helps give your poached eggs a more compact shape because its acid content lowers the cooking liquid's pH factor. This precipitates the setting of the egg whites before they have had a chance to spread too far in the simmering water. Vinegar also blanches the egg whites, enhancing their snowy luster. These benefits are overshadowed, however, when the cook uses other than a quality vinegar or pours the vinegar into the pan with a heavy hand, a bad habit common among short-order chefs. A proper ratio is about 1 teaspoon of vinegar (or 2 teaspoons of lemon juice) per quart of water.
Salt also helps speed the coagulation process, but like vinegar, you must use a sparing amount lest you overpower the egg's flavor. A pinch or two of salt per quart of water should suffice.
** Asian Recipes