Combine sugar, syrup, butter, and vinegar in a 2-quart saucepan. Cook over medium heat, stirring constantly, until sugar dissolves. Cover and cook 2 to 3 minutes to wash down sugar crystals from sides of pan. Uncover and cook, without stirring, until mixture reaches hard crack stage (300°).
Remove from heat; stir in coconut. Spread mixture evenly in a buttered baking sheet. Cool completely; break into pieces. Store in an airtight container.