1Coat a large heatproof bowl with butter, vegetable oil, or cooking spray and place popcorn in the bowl.
2Place sugar, corn syrup, water, vinegar, and salt in a medium saucepan and stir to combine. Place over high heat, stirring until sugar is dissolved, about 2 minutes. Bring to a boil and cook until mixture registers 260°F on a candy/fat thermometer, about 5 to 7 minutes. Remove from heat and stir in butter and vanilla until melted and smooth.
3Immediately drizzle sugar mixture over popcorn and stir continuously with a rubber spatula, scraping the bottom of the bowl, until popcorn is thoroughly coated and cool enough to handle, about 3 minutes.
4Using buttered or oiled hands, tightly press mixture (it may still be a bit warm) into 3-inch rounds. Place on waxed or parchment paper to cool completely, about 15 to 20 minutes.