Place popped popcorn into a large brown paper bag, set aside
In a 2 quart casserole dish or other heat-proof glass dish, combine the brown sugar, butter, corn syrup, salt and vanilla. Heat in microwave for 3 minutes, then take out and stir until well blended. Return to microwave, and cook for 1 1/2 minutes. Remove from microwave and stir in the baking soda until blended
Pour the syrup mixture over the popcorn in the bag. Roll down the top of bag once or twice to close the bag and shake to coat corn. Place bag into the microwave and cook for 1 min and 10 seconds, remove, shake and flip the bag over , and return to microwave for another 1 min 10 seconds.
Dump the popcorn onto waxed paper and let cool until coating is set. store in air tight container