Add 1 tablespoon of the oil to a 4 quart saucepan, and heat over high heat. When oil is hot, add 1/2 cup of popping corn. Keep pan moving constantly. When corn stops popping, remove from heat. Repeat until all corn has been popped. Place popped corn into a buttered bowl.
In a 3 quart saucepan, combine sugar, salt, butter, dark corn syrup, and water. Stir until sugar dissolves. Stir in vinegar. Boil sugar mixture to hard ball stage, 248 degrees F (118 degrees C). Remove caramel from heat. Stir in soda. Beat to thoroughly dissolve the soda.
Pour hot caramel over popcorn, and quickly stir into popcorn; work quickly for the caramel sets up fast. Spread out on wax paper to cool. Store in a tightly covered container.