Heat oven to 250 degrees F. Butter a large roasting pan.
Combine popped corn and nuts in the prepared pan, mixing well. Place in the oven while preparing the glaze.
In a large heavy saucepan over medium heat, combine the brown sugar, corn syrup, butter, orange zest and salt. Bring to a boil, stirring constantly until the sugar dissolves. Boil for 4 minutes without stirring.
Remove from the heat and mix in the vanilla and baking soda. Gradually pour the glaze over the popped corn mixture, stirring to coat well.
Bake until dry; stirring occasionally, approximately 1 hour.
Remove from the oven. Using a metal spatula, free the popcorn from the bottom of the pan. Let cool completely in the pan.
Break into clumps.
Store in an airtight container at room temperature for up to 1 week.