Butter bottom of a heavy 3-quart saucepan. Cook brown sugar and next 3 ingredients in saucepan over low heat, stirring constantly, until sugars dissolve; stir in pecans. Bring to a boil over medium heat; cook, stirring occasionally, 6 to 8 minutes or until a candy thermometer registers 238° (soft ball stage). Remove from heat.
Stir in vanilla; let stand 3 minutes. Beat with a wooden spoon 3 minutes or until mixture begins to thicken. Working rapidly, drop by tablespoonfuls onto wax paper. Let stand until firm.