Place popcorn in a lightly greased large roasting pan; set aside.
Combine butter, brown sugar, corn syrup and salt in a heavy saucepan. Bring to a boil over medium heat; cook 5 minutes. Remove from heat. Add vanilla and baking soda; stir well and pour over popcorn. Mix well.
Bake at 250 degrees for one hour, stirring occasionally. Let cool; break up and store in an airtight container.