Add the popped popcorn to a large paper grocery sack that hasn't been used (make sure it's clean).
Grease a baking sheet with plenty of nonstick spray.
In a 3 quart saucepan, combine brown sugar, butter, salt, and corn syrup. Bring to a boil over high heat, stirring frequently.
When the mixture is at a rolling boil, set a timer for 1 minute.
Remove from heat. Stir in 1/2 teaspoon baking soda. It's going to bubble and foam up, be ready!
Add the vanilla and stir.
Immediately pour the caramel over the popcorn in the bag. It's nice if you have someone around to help you hold the bag open, but I managed it on my own. Stir the caramel in with the popcorn a little bit.
Fold the bag down several times. Place the bag in the microwave and microwave for 1 minute on high heat.
Take it out and shake the heck out of it! Keep the top folded obviously.
Put it back in (top folded) and microwave for 1 more minute.
Shake it like a polaroid.
Pour the popcorn out onto the prepared baking sheet, breaking up any chunks.
Let cool for a couple minutes, then stuff your face!