Combine sugar, margarine & corn syrup in a large glass bowl. Microwave on High for 5 minutes.
Stir until the sugar is dissolved. Slowly stir in sweetened condensed milk.
Microwave on high for 12-15 minutes. Stirring every 3 minutes and giving it a quick turn in the microwave. Use a clean spoon every time. I check the "ball stage" after 6 minutes in a cup of ice cold water.
Add in the vanilla.
Butter a 9 x 13 glass dish and pour hot caramel and refrigerate until set up. Cut into squares and wrap in wax paper.