In a large saucepan over medium heat, combine brown sugar, butter, corn syrup, salt, and vanilla. Bring to a boil, then remove from heat.
Stir in the baking soda until fully incorporated.
Place the popcorn in a large, lightly greased bowl. Pour the caramel mixture over the popcorn and gently fold to coat.
Spread the coated popcorn onto the prepared baking sheet in an even layer.
Bake for 60 minutes, stirring occasionally, until the caramel is set and the popcorn is crisp.
Let cool completely before serving.