Place popcorn in paper bag, set aside. Place butter in 1 1/2 quart dish. Microwave at high for 1 minute or until melted. Stir in sugar, syrup, and salt; cover and microwave at high for 4 minutes. Remove from oven, stir in soda and vanilla. Pour over popcorn; stir until all is coated and no sugar mixture remains in bottom of dish.