Make the dressing: Whisk the vinegar, 1/4 cup water, the lime juice, fish sauce, sugar, ginger, scallions and chile pepper in a medium bowl; set aside.
Make the noodle salad: Cook the noodles as the label directs; drain and set aside. Whisk 2 tablespoons vegetable oil, the fish sauce, sugar and lemongrass in a large bowl; add the broccoli and toss to coat.
Heat the remaining 2 teaspoons vegetable oil in a large cast-iron skillet over medium-high heat. Working in batches if needed, add the broccoli in a single layer and cook until charred on one side, about 4 minutes. Flip and continue cooking until crisp-tender, about 2 more minutes.
Assemble the bowls: Divide the noodles among 4 shallow bowls; top with the broccoli, cucumber, celery, lettuce and mint. Drizzle with the dressing and top with fried onions, peanuts and the reserved scallion greens.