Bring a large pot of water to a boil. Cook the vermicelli noodles according to package instructions (3–5 minutes), then drain and rinse under cold water.

While the noodles cook, prepare the vegetables: slice cucumbers, julienne carrots, chop green onions, and mince chili if using.

In a small bowl, whisk together fish sauce, lime juice, sugar, minced garlic, and sambal to make the nuoc cham dressing.

In a large mixing bowl, combine cooled vermicelli noodles, bean sprouts, and prepared vegetables. Toss well.

Drizzle nuoc cham dressing over the salad and toss to combine. Serve immediately.