Combine sugar, syrup, water, and pecans in heavy 3-quart saucepan.
Stir over medium heat until sugar is dissolved and mixture comes to a boil. Cook, stirring occasionally, until mixture reaches soft-ball stage (small amount forms soft ball when dropped into cold water).
Remove saucepan from heat; add margarine and vanilla.
Allow candy to cool.
Whip until mixture gradually changes to lighter color and becomes creamy.
Drop by tablespoons onto buttered cookie sheet (Push mixture from tablespoon with a teaspoon to hasten dropping before praline becomes too firm to shape).