In a pot, combine vinegar, sugar, and ½ cup water. Bring to a boil over medium-high heat. Remove from heat and add the red onion slices. Let cool completely.

Prepare your grill for medium-high direct-heat cooking. Mix the smoked paprika, cumin, garlic powder, and ¼ teaspoon salt in a small bowl. Rub this spice mixture evenly over the steak.

Lightly coat the bell peppers with cooking spray.

Grill the steak, turning once, for 4-5 minutes per side for medium-rare, or until your preferred doneness. Let the steak rest for 5 minutes before slicing it thinly.

Grill the peppers, turning once, until they are crisp-tender, about 3 minutes per side. Allow them to cool slightly, then cut into ½-inch wide strips.

Heat a dry nonstick skillet over medium heat. Warm the tortillas by cooking each for about 1 minute per side, flipping once.

In a bowl, mash the avocado with a fork, adding lime juice and the remaining ⅛ teaspoon salt until smooth.

Lay out the warmed tortillas and place a layer of lettuce on each. Evenly distribute the sliced steak, grilled peppers, and pickled onions among the tortillas. Add a dollop of the mashed avocado mixture to each. Tightly roll up the tortillas to enclose the filling, then cut them in half diagonally.