Fill a large pot with lightly salted water and bring to a rolling boil; stir in rice vermicelli and return to a boil. Cook pasta uncovered, stirring occasionally, according to packaging instructions, 5 to 6 minutes, depending on thickness of noodles. Rinse with cold water and drain well.
Transfer vermicelli to a bowl. Add pomelo, carrot, cucumber, scallion, and cilantro.
Stir fish sauce, water, Thai sweet chili sauce, brown sugar, and rice vinegar together in a microwave-safe bowl. Microwave for 20 seconds to dissolve the sugar. Whisk slightly and pour dressing over vermicelli mixture. Toss to combine. Serve immediately.