1. In a bowl, combine sugar and flour.
2. In a 2- to 3-quart pan over medium-high heat, stir bacon often until browned and crisp, 8 to 10 minutes. With a slotted spoon, transfer to paper towels to drain. Discard all but 3 tablespoons fat from pan.
3. Add sugar-flour mixture to drippings in pan over medium-high heat. Stir for 1 minute, then add 1 1/2 cups water and the vinegar. Stir until mixture is boiling, 2 to 3 minutes.
4. In a 5- to 6-quart pan over high heat, cook macaroni in boiling water, stirring occasionally, until just tender to bite, 8 to 10 minutes. Drain and return to pan. Stir vinegar mixture into macaroni. Let cool.
5. Just before serving, stir bacon, celery, and green onions into macaroni. Season generously with salt and pepper. For a looser texture, stir in about 1/2 cup water. Pour into a serving bowl.