Pop the bags of popcorn and pour into two (approximately 12″ x 16″) greased baking sheets. Remove the un-popped kernels. Divide the nuts evenly over both sheets. In a medium pot, melt the butter. Stir in brown sugar, corn syrup, and salt. Bring to a boil, stirring constantly. Boil for 5 minutes. Remove from heat and stir in the baking soda and vanilla (the mixture will foam). Pour over the popcorn and nuts. Bake at 250ºF for about 1 hour, stirring every 15 minutes. Remove from the oven if the coating begins to look dull. Cool completely. Break into pieces and store in an airtight container.