HEAT oven to 250 degrees F. Coat a 15x10-inch jelly roll pan or two 13x9-inch baking pans with no-stick cooking spray. Place popcorn in extra-large bowl. Remove unpopped kernels.
MELT shortening in 2-quart saucepan. Stir in brown sugar, corn syrup and salt. Bring to a boil over medium heat, stirring occasionally. Boil without stirring for 4 minutes. Remove from heat. Stir in vanilla and baking soda. Evenly pour over popped corn, quickly stirring until popcorn is evenly coated. Place in prepared pan.
BAKE 45 minutes, stirring every 15 minutes. Cool. Break into bite-size pieces.