In a bowl, combine sugar and flour.
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.
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.
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.
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.