Preheat oven to 250°F. Rub a large roasting pan generously with butter or spray with cooking spray.
Spread the popcorn in the roasting pan and stir in the nuts.
Melt 4 tablespoons of butter in a saucepan set over medium heat. Stir in brown sugar, maple syrup, and salt. Bring to a boil, and boil for 2 minutes, stirring constantly.
Remove from heat and carefully stir in vanilla and baking soda. Mixture should bubble up into a light brown foam. Pour over popcorn.
Using a heat proof scraper or a wooden spoon, fold the sugar mixture over the popcorn until it is coated.
Bake on lower rack of oven for 45 minutes, stirring and breaking up clumps with a spoon every 15 to 20 minutes.
Remove from oven. At this point it won't be completely crisp, but will crisp as it cools. Let cool completely.
Makes about 12 cups