Pour popcorn into a large roasting pan or lasagna pan.
Melt margarine in a saucepan over medium heat. Stir in brown sugar, corn syrup, and salt. Bring to a boil, stirring constantly. Remove from heat; stir in vanilla extract and baking soda until mixture is light and foamy. Pour over popcorn and mix well.
Bake in the preheated oven, stirring every 15 minutes, until a cooled piece of popcorn is crunchy, about 1 hour.
Line a flat work surface with waxed paper. Spread popcorn on waxed paper and let cool.