Preheat oven to 300°F. In a small saucepan, combine brown sugar, margarine, and ¼ teaspoon of the salt; cook and stir over medium heat just until boiling and sugar is dissolved. Stir in vanilla.
Place popcorn in a shallow roasting pan. Drizzle brown sugar mixture over popcorn; toss to coat. Bake, uncovered, for 20 minutes, stirring once. Sprinkle with the remaining ¼ teaspoon salt. Transfer to a large piece of foil or a large roasting pan; let cool for 1 hour. Immediately place in an airtight container; cover and store at room temperature for up to 2 days.
Tips: Sugar Substitute: Choose Splenda® Brown Sugar Baking Blend. Follow package directions to use product amount equivalent to ½ cup brown sugar.
Nutrition per serving with substitute: same as above, except 79 cal., 131 mg sodium, 11 g carb.
To make ahead: Store airtight at room temperature for up to 2 days.