Preheat the oven to 250 degrees F. Line 2 baking sheets with parchment paper; set aside. Place the popcorn and peanuts in a very large bowl or pot. Lightly toss together to combine.
In a heavy saucepan, melt the butter over medium heat. Stir in the corn syrup, brown sugar, and salt. Increase the heat to medium-high and allow the mixture to come to a boil, stirring frequently. Once the mixture has reached a boil, let it bubble for 5 minutes, without stirring. Remove the pan from the heat and stir in the baking soda and vanilla. Be careful because the caramel will bubble up. Be sure to not let the caramel pour out of the pan; you can stir the caramel to tame it. Pour the caramel over the bowl of popcorn-peanuts and toss to completely coat the popcorn.
Divide the popcorn-peanut mixture between the prepared baking sheets and spread in an even layer. Bake for about 45 minutes, being sure to toss and stir every 10/15 minutes, until it is completely crispy.
Once crispy, remove the baking sheets from the oven and allow the cracker jacks to cool completely. Carefully break apart the cracker jacks into large bite-size pieces. Place the cracker jacks in a sealed plastic baggie until ready to serve/eat.
