Pop the popcorn. Air pop, stove top, or purchase an unseasoned pre-popped variety. Place into a clean paper bag and shake to get all of the unpopped kernels down to the bottom. Scoop out the popcorn and place into a large roasting pan. Set aside.
Preheat oven to 250°.
In a medium-sized saucepan, combine margarine, corn syrup, brown sugar, and salt and heat to a boil. Keep at a rapid boil for 6 minutes, stirring constantly.
Remove from heat and add vanilla and baking soda. It will foam up. Pour caramel sauce over the popped popcorn and fold the corn, scooping from underneath and turning over until all the pieces are coated with sauce.
Place roasting pan into oven for 45 minutes to 1 hour, stirring every 15 minutes.
Turn out hot caramel corn onto a large, clean tea towel and allow to dry and get crispy. Store in an airtight container.