Grease a cookie sheet; set aside. Combine sugar and corn syrup in a 2-quart microwave-safe bowl. Microwave on high for 4 minutes, stirring halfway through cooking time. Stir in peanuts and microwave on high for 2 minutes more. Stir in butter and vanilla: microwave for 30 seconds. Stir in baking soda until light and foamy; immediately pour onto the prepared cookie sheet and tilt the pan to spread. Cool completely. Break into pieces, and store in an airtight container.