In a large bowl, combine the popcorn, cereal, pretzels and pecans.
In a saucepan over medium heat, combine the brown sugar, butter and corn syrup. Cook and stir until mixture comes to a boil. Reduce heat to medium low; cook without stirring for 5 minutes.
Remove saucepan from heat and stir in vanilla and baking soda. Pour over popcorn mixture and toss until evenly coated.
Bake at 300 degrees F (150 degrees F) for 30 minutes, stirring after 15 minutes. Transfer to large piece of parchment paper and cool completely. Break into chunks once cool.
