Lightly butter 2 baking sheets. Set out candy thermometer. Mix together in a heavy 3 quart saucepan having a tight fitting cover the sugar, corn syrup and water. Stir over low heat until sugar is dissolved. Increase heat and bring to boiling. Cover saucepan and boil gently for 5 minutes. Uncover and put candy thermometer in place. Continue cooking without stirring until mixture reaches 234 degrees. During cooking, wash crystals from sides of pan from time to time. Mix in peanuts and butter. Cook over low heat, stirring frequently, until mixture reaches 300 degrees. Remove from heat and remove thermometer. Add, mixing well, the baking soda and vanilla extract. Pour onto baking sheets, spreading as thinly as possible. As soon as candy is cool enough to handle, wet hands in water and stretch candy as thinly as desired. Turn candy over and cool completely. When cool and firm, break candy into pieces. Store in tightly covered container. Yield: about 2 pounds peanut brittle.