Step 1) Combine oil, vinegar, lemon juice, oregano, cumin and garlic salt in a medium bowl. Pierce steak evenly with a fork. Place steak in a resealable plastic bag; place onion and pepper in a separate bag.
Step 2) Pour 1 cup marinade over steak; close bag. Pour remaining marinade over vegetables; close bag. Marinate both bags 8 hours or overnight in refrigerator.
Step 3) Remove steak from bag; discard bag and remaining marinade. Grill or broil 7 to 10 minutes on each side or to desired doneness. Pour contents of vegetable bag into large skillet and stir-fry 5 minutes or until vegetables are tender crisp and liquid is evaporated.
Step 4) Slice steak thinly across the grain; add to vegetables. Fill warm tortillas with beef mixture. Garnish with sour cream, avocado, cilantro and lime wedges, if desired.