In a 4-quart Dutch oven heat and stir the brown sugar, granulated sugar, margarine, and salt over medium heat until just boiling and sugar is dissolved. Stir in vanilla. Add popcorn and toss until coated.
Place coated popcorn in a shallow roasting pan. Bake, uncovered, in a 300 degrees F oven for 15 minutes, stirring once. Transfer to a large piece of foil or a large roasting pan to cool.