Combine sugar and water in a heavy saucepan, and place over medium-low heat. Cook 13 minutes or until sugar dissolves. (Do not stir.) Cover, increase heat to medium, and boil 1 minute. (This will dissolve any sugar crystals clinging to sides of pan.) Uncover and boil 10 additional minutes or until amber or golden. (Do not stir.)
Remove from heat; let stand 1 minute. Carefully add margarine, stirring until it melts. Gradually add milk, stirring constantly. (Caramel will harden and stick to spoon.) Place pan over medium heat, and cook, stirring constantly, 3 minutes or until caramel melts and mixture is smooth. Remove from heat; stir in vanilla and salt. Pour sauce into a bowl; serve warm or chilled. Store in an airtight container in the refrigerator.