Melt butter in a large saucepan over medium heat. Add corn syrup and brown sugar; bring to a boil, stirring constantly. Boil for 5 minutes, stirring constantly. Remove from heat and stir in vanilla.
Place popcorn and pretzels in a large bowl. Pour caramel mixture over popcorn and pretzels; stir until evenly coated. Let cool for 10 minutes.
Shape mixture into 1 1/2-inch balls with buttered hands. Place on waxed paper to cool completely.