Preheat oven to 300 degrees F. Spray a 17x12x2 inch baking dish with nonstick cooking spray and spread out popcorn. Place in preheated oven to keep warm while making syrup.
Generously spray a 2 quart heavy pot with nonstick cooking spray and add everything but the vanilla. Cook and stir over medium high heat until sugar dissolves and mixture is boiling, about 6 minutes.
Clip a candy thermometer to side of pan, lower heat to medium, and continue boiling at a steady moderate pace, stirring occasionally, until thermometer reads 250 degrees F. (hard ball stage). Adjust heat if necessary to keep mixture boiling at an even rate.
Remove from heat, remove thermometer, and stir in vanilla. Pour mixture over hot popcorn and gently stir to coat. Allow popcorn to cool until easy to handle.
Spray your hands with nonstick cooking spray and quickly roll popcorn into 2 1/2 inch diameter balls (you should get about 20). Wrap in plastic wrap for up to 1 week.