Bake pecans in a single layer in a shallow pan 8 to 10 minutes or until toasted and fragrant, stirring halfway through. Cool completely (about 15 minutes).
Meanwhile, bring brown sugar and next 4 ingredients to a boil in a heavy Dutch oven over medium heat, stirring constantly. Boil, stirring occasionally, 6 to 8 minutes or until a candy thermometer registers 236 (soft ball stage).
Remove sugar mixture from heat.
Let sugar mixture stand until candy thermometer reaches 150 (20 to 25 minutes). Stir in vanilla and pecans using a wooden spoon; stir constantly 1 to 2 minutes or just until mixture begins to lose its gloss. Quickly drop by heaping tablespoonfuls onto wax paper; let stand until firm (10 to 15 minutes).
