Heat oil in heavy large skillet over medium heat. Add bell pepper and onion and sauté until tender, about 5 minutes.
Transfer to bowl. Add steak to skillet and cook until no longer pink, about 2 minutes. Add jalapeño, cumin, chili powder and salt and pepper to taste.
Transfer to heated bowl. In a medium sized bowl combine the corn, tomatoes, avocado, cilantro, vinegar and oil. Warm tortillas over gas flame or electric burner until they begin to color.
Transfer to a napkin-lined basket. To serve, have diners assemble their own tacos.