In a blender, combine the fish sauce, vegetable oil, brown sugar, garlic, shallot, 2 teaspoons salt, a few grinds of black pepper and 1/4 cup cold water. Blend until smooth. Place the steak in a 1-gallon resealable plastic bag. Pour the marinade into the bag, seal, and toss to coat. Refrigerate for at least 2 hours.
Meanwhile, combine the granulated sugar, vinegar, 1 tablespoon salt and 1 cup cold water in a small saucepot. Bring to a boil and then pour the hot pickling liquid over the carrots and jicama. Let sit until cooled to room temperature. Use at room temperature or refrigerate, covered, for later use.
Prepare a grill for high heat. Remove the steak from the marinade and pat dry. Grill the steak for 3 to 4 minutes per side for medium. Remove from the grill, let rest for 5 minutes, and then cut into 1/2-inch pieces.
Warm the tortillas in a dry saute pan or in a 350-degree oven, wrapped in a foil pouch, for 10 minutes (you can also heat them over a gas burner until charred in spots). Spread some of the mayonnaise on one side of each tortilla. Top with some steak, cucumbers, chiles and pickled carrots and jicama. Sprinkle with cilantro and cotija.