Lightly grease two 12"x17" baking sheets. Set aside. Preheat oven to 250 degrees.
In a medium saucepan, melt butter, corn syrup, and brown sugar. Bring to a boil. Cook and stir for about 1 minute. Remove from heat, and stir in baking soda and vanilla. Mixture will foam up. Stir well.
Pour caramel over popcorn and stir well. Spread in baking sheet. Bake in 250 degree oven for about 30 minutes, stirring every 10 minutes. Spread on waxed paper to cool. Store in an airtight container at room temperature.