Remove and discard unpopped kernels from popped popcorn. In large microwavable bowl, mix cereal, popcorn and peanuts; set aside.
In medium microwavable bowl, microwave butter, brown sugar, corn syrup and vanilla uncovered on High about 2 minutes or until mixture is boiling, stirring after 1 minute. Pour over cereal mixture, stirring until evenly coated.
Microwave uncovered on High 5 to 6 minutes, stirring and scraping bowl after every minute. Spread on waxed paper to cool, about 15 minutes, stirring occasionally to break up. Store in airtight container.