Move oven rack to the middle position. Preheat oven to 250 degrees F. Grease a large, disposable aluminum pan with nonstick spray. Pour the popcorn into the pan; set aside.
In a large saucepan over medium-high heat melt the butter. Add the sugar, corn syrup, and salt. Bring to a boil, then reduce heat to simmer. Simmer the mixture while stirring often, until slightly thickened, about 3-5 minutes. Remove from heat, and stir in the vanilla and baking soda. The baking soda will foam, keep stirring until the foaming subsides. Add the peanuts and pour everything over the popcorn, tossing to coat evening. This will take a couple of minutes to do.
Bake the popcorn, stirring every 15-20 minutes, until it turns a deep golden color and the caramel has set, 60-90 minutes. Spread wax paper over your counter and pour the mixture out onto the wax paper. Cool the popcorn to room temperature. Enjoy!