Trim 2 pounds flank or skirt steak and cut into 3 equal portions.  Whisk vinegar, chili powder, oil, 2 teaspoons salt, and garlic powder in a 9- by 13-inch baking dish. Add steak and turn to coat. Cover and marinate in the refrigerator for at least 1 hour and up to 24 hours.

Make salsa about 20 minutes (and up to 2 hours) before cooking the meat:  Chop 1 pound of tomatoes, enough onion to yield 1/2 a cup onions, the avocado, jalapeno and cilantro. Gently combine tomatoes, jalapeños (or chiles) to taste, onion, cilantro, avocado, lime juice, and the remaining 3/4 teaspoon salt in a medium bowl.

Position rack in upper third of oven; preheat broiler.  Place the steak on a rimmed baking sheet (discard the marinade). Broil 3 to 4 minutes per side for medium.

Transfer to a clean cutting board and let rest for 5 minutes. Thinly slice against the grain. Serve the steak wrapped in warm tortillas and topped with the salsa and cheese.