Combine the sugar, margarine, and milk. Bring to a rolling boil, stirring constantly. Boil 5 minutes (I usually boil about 7) over medium heat or until a candy thermometer reaches 238 degrees (or until a small amount of the mixture forms a soft ball in water), stirring constantly. Remove from the heat, stir in chocolate pieces until melted. Add the marshmallow cream, nuts, and vanilla, and beat until well blended. Pour into a greased 9-inch by 13-inch pan. Cool, and cut into squares. Makes 3 pounds.