Place popcorn and peanuts in a large mixing bowl; set aside.
Melt butter in a large heavy saucepan. Stir in brown sugar and next 3 ingredients; bring to a boil over medium heat, stirring constantly.
Boil 5 minutes, stirring occasionally. Remove from heat, and stir in vanilla.
Pour mixture over popcorn mixture; stir until evenly coated. Pour into 2 large lightly greased roasting pans or 4 (13- x 9-inch) baking pans, spreading into a thin layer.
Bake at 250° for 45 to 50 minutes, stirring every 15 minutes. Cool in pans on wire racks.
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