Pop the corn, pick out the old maids, and keep warm with nuts in a roasting pan in the oven 275*. Butter two sheet pans.
Stir the sugar, butter, corn syrup and salt together in a 4 - 5 cup heavy bottomed pan over low heat until the sugar melts. Increase heat to medium and constantly stir until the mix reaches a rolling boil. Boil 4 minutes and remove from heat. Add in the baking soda and vanilla and stir to make sure the soda is thoroughly incorporated.
Pour caramel over the warm popcorn and stir gently (so you don't break the popcorn) until it is completely covered in caramel. Divide between the two sheet pans and bake for 40 minutes, stirring every 15 minutes. Don't skip the stirring.
Cool on waxed paper to room temperature. Store in two zip lock gallon baggies.