Put water in the bottom of a double boiler up to the water line as marked. Bring to boiling point over medium heat. Measure the 2-1/4 cups water into the top part of the double boiler. Cover and bring to boiling point. Add rice, salt, and vinegar to boiling water in the top part of the double boiler. Cover tightly. Time for 25 minutes without peeking. Uncover and stir around once with a fork to mix in any remaining liquid. Too much stirring makes rice grains stick together. Taste to make certain that rice is tender. Makes 3 to 4 cups cooked rice.