In medium bowl, combine flour and sugar; cut in margarine until crumbly. Press firmly on bottom of 13x9 inch baking pan. Bake 15 minutes.
Meanwhile, in medium bowl, beat sweetened condensed milk, egg and vanilla. Stir in chips and pecans. Spread evenly over crust. Bake 25 minutes or until golden brown. Cool. Cut into bars. Store covered in refrigerator.