In a small bowl, whisk together the vinegar, orange juice, oil, salt, and freshly ground pepper until well blended. Cook chicken breasts in boiling, salted water 10 minutes, or until chicken is no longer pink inside. Remove chicken and cool 5 minutes on a cutting board; slice into long strips. Transfer warm chicken to a bowl and toss with a few tablespoons of the dressing. Season to taste with salt and freshly ground pepper. With a sharp knife, remove orange peel and white pith from oranges; slice crosswise into thin rounds. In a large bowl, toss onion and lettuce with remaining vinaigrette. Divide between bowls and top salads with the chicken, oranges, and avocado slices.