On a large serving platter or six individual salad plates, arrange the mixed salad greens, grapefruit sections, and avocado slices.
For dressing, in a small bowl, whisk together vinegar, oil, the water, sugar, and salt. Drizzle dressing over the salad.
To make ahead: Prepare dressing as directed. Cover and chill for up to 24 hours. To serve, prepare salad. Whisk dressing; drizzle it over the salad.
2 fat, 1 vegetable, 1/2 fruit