Mix margarine, sugar and evaporated milk in 2 1/2 to 3 quart pan. Bring to a full rolling boil over medium heat. Stir constantly. Boil for 5 minutes or to 234 degrees on candy thermometer, soft ball stage, stirring constantly to prevent scorching. Remove from heat. Stir in chocolate pieces gradually until smooth. Add the marshmallow crème, vanilla and pecans and mix well. Pour into buttered 9 x 12 inch dish. Cool at room temperature. Cut into squares. Makes 3 dozen.