"It's worth seeking out flat-iron steak for this. Butchers used to set it aside for their own enjoyment, but it's now gained popularity with smart home cooks, too. Like most chuck steaks, it's full of flavor and reasonably priced."
Combine vinegar, water, 2 tablespoons sugar, fish sauce, and red pepper flakes in a glass, ceramic, or stainless steel bowl; stir until sugar is dissolved. Add carrots and daikon radish. Put a plate on top to weigh down vegetables and refrigerate for 4 hours or overnight.
Combine soy sauce, 1 tablespoon sugar, and garlic in a shallow baking dish; stir until sugar is dissolved. Add steak, pressing down until steak is covered in marinade, and refrigerate for 4 hours or overnight.
Heat a ridged grill pan over high heat; add oil. Remove steak from marinade and discard marinade. Grill steak until desired doneness is reached, 6 to 10 minutes per side. Transfer steak to a cutting board and cool. Slice across grain into 24 thin slices.
Drain carrot and daikon radishes, reserving 1/2 cup pickling liquid.
Fill a shallow bowl with warm water. Dip 1 rice paper sheet into water to soften, about 5 seconds. Transfer sheet to a work surface. Arrange 2 pieces of lettuce over the half of wrapper closest to you. Top each lettuce leaf half with 2 slices of steak. Add about 1/4 cup carrot and daikon pickles and a few cilantro sprigs. Starting with bottom edge, begin to roll up rice paper sheet tightly around filling. After 1 roll, fold in ends of rice paper like a burrito, then finish rolling. Repeat with remaining wrappers and filling. Diagonally cut rolls in half crosswise and arrange on a serving platter.
Whisk peanut butter, reserved pickling liquid, and sriracha hot sauce together in a bowl; serve on the side with rolls.
Pickles can be made 2 days ahead and kept chilled, covered. Steak can be cooked, cooled, and chilled 1 day ahead. Peanut sauce can be made 2 days ahead and kept chilled, covered. Rolls can be assembled up to 1 day ahead and kept chilled, covered with damp paper towels and plastic wrap.
The nutrition data for this recipe includes the full amount of the pickling liquid and the marinade for the steak. The actual amount consumed will vary.
Per Serving: 179 calories;9.5 g fat;
14.4 g carbohydrates;
10.2 g protein;
26 mg cholesterol;
836 mg sodium.