Combine Worcestershire, vinegar, cumin, garlic, black pepper, salt and red-pepper flakes in a measuring cup; whisk to combine.
Pour half of marinade into glass dish or plastic bag. Add steak. Marinate in refrigerator for 2 hours.
Add chili powder and oil to marinade in measuring cup.
Heat grill or broiler. Remove meat from marinade and discard marinade in bag.
Grill or broil meat for 12 to 15 minutes, turning once, until internal temperature registers 150 degrees F. Remove to cutting board to rest.
Meanwhile, place large nonstick skillet over medium-high heat. Add reserved marinade from measuring cup; heat until sizzling. Add sweet pepper and onion; cook for about 3 minutes. Add mushrooms and zucchini; sauté for 4 minutes.
Slice steak thinly across grain. Add steak to vegetables in skillet; toss to coat. Remove from heat. Use to fill flour or flavored tortillas, or taco shells. Makes 4 cups.