Place popped popcorn in a large microwave-safe bowl.
In another microwave-safe bowl, combine brown sugar, corn syrup, butter, and salt. Microwave on high for 2-3 minutes, stirring every minute, until melted and smooth.
Stir in baking soda and vanilla extract into the caramel mixture.
Pour caramel mixture over the popcorn and stir to coat evenly.
Spread the coated popcorn on a baking sheet to cool and set.