Place pecans in a large bowl. Set aside.
Combine brown sugar, maple syrup, and butter in a small heavy saucepan; bring to a boil over medium-high heat, stirring often. Reduce heat to medium, and cook 2 minutes, stirring often. Remove from heat, and stir in vanilla.
Drizzle sugar mixture over pecans, tossing to coat. Add popcorn, 2 cups at a time, stirring until well coated.
Spoon popcorn mixture onto a buttered 15" x 10" jellyroll pan. Bake at 250° for 1 hour, stirring mixture every 15 minutes with a large spoon. Cool completely. Store in an airtight container.