Preheat oven to 300 degree F. Discard any unpopped popcorn kernels. In a 17x12x2-inch roasting pan combine the popped corn, pretzels, and peanuts; set aside.
In a medium saucepan melt butter. Remove saucepan from heat. Stir in brown sugar, corn syrup, molasses, and salt. Cook and stir over medium heat until mixture boils.
Clip a candy thermometer to side of pan. Cook, stirring occasionally, until thermometer registers 238 degree F (soft-ball stage), about 5 minutes. Remove saucepan from heat. Immediately stir in baking soda and vanilla. Pour over popcorn mixture, stirring to coat.
Bake popcorn mixture in preheated oven, uncovered, for 25 minutes, stirring twice. Remove pan from oven. Turn mixture onto a large piece of foil. Cool completely. Break into pieces. Store in an airtight container at room temperature up to 1 week. Makes 14 cups.