Place popcorn in a large microwave-safe glass bowl, set aside.
In another glass bowl, microwave the butter until melted, about 45 seconds. Stir in brown sugar and corn syrup. Microwave on high until mixture boils, 1 to 3 minutes, stirring once, then on high for 3 minutes without stirring. Remove bowl and stir in soda and vanilla.
Pour the caramel mixture over the popcorn and stir until most of the popcorn is coated. Microwave on 70% power for 1 minute. Remove and stir, then microwave at 70% for 1 more minute. Remove and stir again to coat popcorn evenly. Pour popcorn onto wax paper and cool completely, about ten minutes. Break apart and serve, or store in an airtight container.