Preheat oven to 250 F.
Pop the corn and add it to a very large bowl with the peanuts.
Prepare two large rimmed baking sheets with parchment paper or spray with cooking spray. (These steps must be done before starting the caramel syrup).
In a saucepan, add the brown sugar, butter, syrup and salt. Over medium heat, bring the mixture to a boil while stirring.
Once it reaches a boil, stop stirring and continue boiling for 5 minutes.
Remove from heat and stir in vanilla and baking soda.
Pour over popcorn and peanuts. Mix well and divide evenly between the two baking sheets.
Bake for 1 hour, stirring occasionally.
Allow the popcorn to cool before storing in airtight containers.