Heat oven to 250. Grease 14x10" roasting pan. Pop the popcorn. Put nuts and popcorn in a very large bowl. Bring brown sugar, corn syrup and butter to boil, stirring until sugar is dissolved. Reduce heat and cook for 5 minutes. Remove from heat; stir in butterscotch chips, vanilla, baking soda and salt until blended and smooth. Working quickly and using two wooden spoons pour syrup over popcorn and nuts, stir to coat thoroughly. Pour mixture into pan; bake 45 minutes, stirring occasionally. Remove from oven, cool mixture in pan about 15 minutes. Turn mixture out of pan onto foil to cool completely. Break popcorn into smaller pieces; store in airtight containers in cool dry place up to 2 weeks. Makes about 4 quarts.