In large microwavable bowl, mix cereal, popcorn and peanuts. Set aside.
In medium microwavable bowl, mix butter, brown sugar, corn syrup and vanilla. Microwave 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 4 to 5 minutes, stirring and scraping bowl after every minute.
Spread on waxed paper until cooled, stirring occasionally to break up. Store in airtight container.