Trim fat from steak. Combine 2 tablespoons vinegar, 2 tablespoons broth, 1/2 teaspoon pepper, and garlic in a large zip-top heavy-duty plastic bag. Add steak; seal bag, and marinate in refrigerator 1 hour. Remove steak from bag; discard marinade.
Coat a large nonstick skillet with cooking spray, and place over medium-high heat until hot. Add steak; cook 6 minutes on each side or until desired degree of doneness. Cut steak across grain into thin slices.
Combine steak, potato, cucumber, green onions, celery, and parsley in a large bowl; toss well. Combine 1/3 cup broth and the next 5 ingredients (broth through 1/4 teaspoon pepper); stir well. Add to steak mixture; toss well. Serve on spinach-lined plates.