Preheat the oven to 250°F (121°C) and line a rimmed baking sheet with parchment paper.
In a large bowl, combine the popcorn and peanuts.
In a small saucepan over medium-high heat, melt the butter.
Stir in the maple syrup and salt. Bring to a boil and cook until the mixture reaches 230°F (110°C) on a candy thermometer, anywhere from 3 to 7 minutes. The mixture should flow in a steady stream from the spoon.
Remove the caramel from the heat and very carefully stir in the vanilla and baking soda. The mixture will foam up.
Immediately pour the caramel over the popcorn and peanuts and quickly toss with a spatula until coated.
While the mixture is still hot, use the back of the spatula to spread it in the baking sheet in an even layer. Bake, stirring every 15 minutes, until the popcorn is light and crisp, about 45 minutes.
Cool briefly before serving. The maple caramel corn will keep in an airtight container at room temperature for 2 to 3 days.