Melt margarine in 2-quart pan; stir in brown sugar, corn syrup, soda, salt and vanilla. Bring to a rolling boil stirring constantly. Boil without stirring for 5 minutes.
Remove from heat; stir into popcorn mix and toss.
Spread in shallow pan. Bake at 250 degrees for 45 minutes, stirring once or twice.
Remove from oven and turn out on foil to cool
When cooled, break into pieces. Store tightly covered.