Line a baking sheet with parchment. Lightly oil your hands or a scoop.
Place the popcorn in a very large bowl and remove any unpopped kernels.
Heat corn syrup, sugar, and butter in a saucepan over medium heat, stirring until dissolved. Simmer 2 minutes.
Take the pan off heat. Stir in peanut butter and salt until smooth, then mix in vanilla.
Pour the warm mixture over the popcorn. Toss gently with a spatula until evenly coated.
Let cool 2–3 minutes. With oiled hands, form 12 balls, pressing just until they hold.
Set the balls on the baking sheet and let firm up about 15 minutes before serving.