Preheat oven to 250°F. Line a baking tray with parchment paper and toss popcorn and peanuts on tray. Keep warm in oven while preparing caramel.
In a saucepan over medium-low heat, stir brown sugar, butter and maple syrup until butter is melted.
Increase heat to high and boil sugar mixture until a candy thermometer reads 255 F (just past softball stage), about 4 minutes.
While boiling, occasionally brush the sides of the pot with a pastry or silicone brush dipped in cool water. Remove from heat and stir in vanilla extract, vinegar, salt and baking soda (mixture will bubble).
Slowly pour syrup over popcorn and peanuts, gently stirring to coat completely.
Bake until caramel feels dry, stirring occasionally, about 1 hour 20 minutes. Remove from oven. Using metal spatula, scrape mixture from bottom of pan to loosen. Cool completely in pan.
Caramel Peanut Popcorn will keep up to a week, stored in airtight container.