Melt butter in a saucepan over medium heat. Stir in brown sugar, corn syrup and salt. Bring to a boil, stirring constantly. Boil for 5 minutes without stirring; remove from heat. Add baking soda and vanilla; mix well. Place popcorn in an extra-large bowl; pour mixture over popcorn, stirring to coat well. Spread popcorn on lightly greased baking sheets. Bake at 250 degrees for 15 minutes. Stir; bake for an additional 15 minutes. Cool; store in an airtight container.