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.