Preheat the oven to 250F. Line a baking sheet with a silpat or parchment paper before you begin cooking.
Place the popped popcorn into a large, heat-proof mixing bowl. Make sure you have no un-popped kernels in the bowl.
Melt the butter in a medium sized saucepan over medium-high heat. Mix in the sugar, and bring to a boil. Cook for 3-4 minutes, stirring and scraping the bottom of the pan continuously.
Turn off the heat and stir in the vanilla, baking soda, and salt. The mixture will bubble up a lot, this is okay. Continue stirring until the caramel is thick and glossy.
While stirring the popcorn, slowly pour the caramel over the popcorn. This is easiest if you have a partner help you. Stir until all the kernels are coated.
Spread the popcorn onto the baking sheet in an even layer.
Bake for 1 hour, stirring every 15 minutes, and breaking up any large clumps.
Let the caramel corn cool completely before serving.