1. Place popcorn in a large bowl. Line a large baking sheet with waxed paper. Combine sugar, corn syrup and vinegar in a heavy-bottom pan over medium-high heat, stirring until sugar is dissolved. Continue cooking without stirring until mixture reaches 260ºF on a candy thermometer, 12 to 15 minutes. Remove pan from heat and carefully stir in vanilla and butter, mixing until butter has melted.
2. Pour mixture over popcorn and stir to coat. Let stand for 3 minutes, stirring occasionally, until mixture has cooled slightly.
3. Grease hands with cooking spray and form mixture into 12 balls, each 2 1/2 to 3 inches. Press 6 to 10 M&M's into each ball. Allow to cool on lined baking sheet.