Bring first 4 ingredients to a boil in a 3-quart saucepan over medium heat, stirring mixture constantly. Cook, stirring occasionally, 6 to 8 minutes, or until a candy thermometer registers 236° (soft ball stage).
Remove mixture from heat, and add butter (do not stir). Let stand until candy thermometer reaches 150°. Stir in pecans and vanilla, using a wooden spoon; stir constantly until candy begins to thicken.
Drop by heaping teaspoonfuls, working rapidly, onto wax paper. Let stand until firm.