Preheat the oven to 350°. Line a large rimmed baking sheet with a silicone liner or buttered parchment paper. In a large bowl, mix the popcorn and chopped nuts. In a small saucepan, combine the sugar, corn syrup and butter and bring to a boil. Cook over moderately high heat for 2 minutes. Off the heat, whisk in the vanilla, salt and baking soda. Immediately pour the caramel over the popcorn and toss with a rubber spatula coated with vegetable oil spray.
Spread the popcorn on the baking sheet and bake for 2 minutes; stir to coat. Repeat 3 more times, until the popcorn is evenly coated with caramel. Remove from the oven and stir until the caramel hardens, 5 minutes.Transfer to a bowl and serve.