Heat oven to 200°F
Place both bags of puffcorn into a large roasting pan; set aside
Combine the butter, brown sugar, corn syrup and salt in a large saucepan. Cook over medium heat, stirring occasionally, until the mixture comes to a rolling boil. Once it is boiling, keep cooking for 5 minutes, stirring every minute or so
Remove from the heat; stir in the vanilla and then stir in the baking soda. The mixture will turn foamy
Carefully pour the foam over the puff corn and stir until well covered
Transfer to the oven, and cook for 1 hour, stirring every 15 minutes, or until the puffed corn is nice and crisp
Use a spatula to immediately place the caramel corn on to a large baking sheet lined with waxed paper and let cool completely
Break the caramel corn in to bite sized pieces and place in a tightly sealed container to keep fresh