Preheat oven to 250°F.
Toss together popcorn, peanuts, cereal and broken crackers in large roasting pan.
Melt butter in a medium-size saucepan over low heat. Add brown sugar and corn syrup. Bring mixture to boiling, stirring constantly. Gently boil without stirring for 5 minutes. Remove from heat. Stir in baking soda and vanilla until smooth.
Pour corn syrup mixture over popcorn mixture in roasting pan; toss to coat evenly.
Bake in preheated 250°F oven for 1 hour, stirring every 15 minutes. Remove from oven and cool. Break popcorn into small clusters. Store in airtight container for up to 1 week.