Lightly coat the bottom and sides of a 17x12x2-inch baking or roasting pan with nonstick cooking spray. Pop popcorn according to package directions. Place the popped popcorn in prepared pan; remove all unpopped kernels; set aside.
In a heavy medium saucepan, melt butter over medium heat. Stir in brown sugar, corn syrup and salt. Cook and stir over medium heat until mixture boils. Continue boiling at a moderate, steady rate, without stirring, for 5 minutes more.
Remove saucepan from heat. Stir in vanilla and baking soda. Slowly pour caramel mixture over popcorn; stir gently to coat.
Bake in a 250 degree F oven for 1 hour, stirring every 15 minutes. Remove from oven. If you like, spread caramel corn on a large piece of buttered foil to cool or allow to cool in roasting pan. When completely cooled, break apart. Store tightly covered at room temperature for up to 1 week. Makes about 10 cups (10 servings).