Pop popcorn in a hot air popper according to manufacturer's instructions. Transfer to a large bowl; set aside
Combine sugar, corn syrup, butter, and water in a heavy saucepan over medium heat; attach a candy thermometer to the saucepan and cook until mixture reaches 280 degrees F (138 degrees C), about 10 minutes. Off heat, stir in vanilla extract.
Toss popcorn and toffee syrup together until evenly combined.