Mix popcorn, Rice Chex, Cheerios and peanuts in a large bowl. Preheat oven to 250 ° F. Melt butter in a large saucepan; stir in brown sugar and corn syrup. Heat slowly, stirring constantly, to boiling. Cook without stirring another 5 minutes. Remove from heat; stir in soda and vanilla. Drizzle over popcorn mixture and toss until evenly coated. Spoon scramble into a shallow pan and bake, stirring several times, for 1 hour. Cool. Store in a tightly covered container.