1/2 cup white vinegar
2 tablespoon chili powder
2 tablespoon extra-virgin olive oil
2 3/4 teaspoon salt, divided
1 teaspoon garlic powder
2 pound flank or skirt steak, trimmed and cut into 3 equal portions
1 pound tomatoes, chopped
2 fresh jalapeno peppers or serrano chiles, seeded and finely diced
1/2 cup chopped onion
1/2 cup chopped fresh cilantro
1 ripe but slightly firm avocado, diced
Juice of 1 lime
16 5- to 6-inch corn or whole-wheat flour tortillas, warmed
1/3 cup shredded cheese
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: 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.