Line an 8x8x2-inch baking pan with foil, extending foil over edges of pan. Butter the foil; set pan aside.
Butter the sides of a heavy 1-quart saucepan. In the saucepan combine sugar, light corn syrup, water, and vinegar. Cook over medium-high heat to boiling, stirring constantly with a wooden spoon to dissolve sugar. This should take about 5 minutes. Avoid splashing mixture on sides of pan. Carefully clip candy thermometer to side of pan.
Cook over medium heat, stirring constantly, while adding the 1/4 cup butter, two pieces at a time. Continue cooking over medium heat, stirring occasionally, until thermometer registers 300 degree F, hard-crack stage. Mixture should boil at a moderate, steady rate over the entire surface. Reaching hard-crack stage should take 25 to 30 minutes.
Remove saucepan from heat; remove thermometer from saucepan. Stir in vanilla. Pour mixture into prepared pan. Let stand 5 to 10 minutes or until a film forms over surface of candy.
Using a broad spatula or pancake turner, begin marking candy by pressing a line across surface, 1/2 inch from edge of pan. Do not break the film on the surface. Repeat along other three sides of pan, intersecting lines at corners to form squares. (If candy does not hold its shape, it is not cool enough to mark. Let candy stand a few more minutes and start again.)
Continue marking lines along all sides, 1/2 inch apart, until you reach the center. Retrace previous lines, pressing the spatula deeper but not breaking film on surface. Repeat until the spatula can be pressed to bottom of pan along all lines.
Cool completely. Use the foil to lift candy out of the pan; break into squares. Store tightly covered. Makes about 48 servings or 2/3 pound.
Up to 1 month ahead, prepare butterscotch. Store at room temperature in a tightly covered container.
(1 g saturated fat,
3 mg cholesterol,
12 mg sodium,
5 g carbohydrates,
0 g fiber,
0 g protein.