In a very large (4 quart) saucepan, melt the butter over medium heat. Add the brown sugar and next 3 ingredients, stirring constantly until mixture boils. Boil gently, uncovered and without stirring, for 5 minutes. Remove from burner and add baking soda and vanilla. Pour warm mixture over popcorn and toss until coated. If you are using two roasting pans, you can do this directly in the roasting pans. If using rimmed cookie sheets, you might want to divide the popcorn among a few large bowls or put it in a really big one to stir.