In a medium bowl, combine 1 tablespoon fish sauce, soy sauce, brown sugar, 2 teaspoons oil, 1 teaspoon garlic, and black pepper. Add the pork and turn to coat. Cover and refrigerate for 20 to 30 minutes while you prepare the rice and vegetables.
Add the rinsed broken rice and water to a medium saucepan. Bring to a boil over medium-high heat, then cover, reduce the heat to low, and cook for 15 to 18 minutes, until the water is absorbed. Remove from the heat and let stand, covered, for 10 minutes. Fluff with a fork.
For quick pickles, combine the carrot and half of the cucumber in a bowl. Stir together the rice vinegar, 1 tablespoon sugar, kosher salt, and 2 tablespoons warm water until dissolved. Pour over the vegetables and set aside for at least 15 minutes.
Make the dressing by stirring the remaining 1 tablespoon fish sauce, remaining 1 tablespoon sugar, 1/4 cup warm water, lime juice, remaining garlic, and optional chile in a small bowl. Stir until the sugar dissolves.
Heat an outdoor grill or grill pan over medium-high heat. Lightly oil the grates or pan. Remove pork from the marinade, letting excess drip off, and discard the used marinade. Grill pork for 4 to 6 minutes per side, depending on thickness, until browned with light char marks and the center reaches 145°F (63°C). Rest for 5 minutes, then slice across the grain.
Heat the remaining 1 teaspoon oil in a small skillet over medium heat. Add the scallions and cook for 30 to 45 seconds, just until softened. Spoon the scallion oil over the cooked rice.
Divide rice among four bowls or plates. Top with sliced pork, quick pickles, remaining fresh cucumber, tomato wedges, and cilantro if using. Serve the fish sauce dressing on the side for everyone to spoon over their bowl.