Whisk vinegar and sugar in medium bowl to blend. Add onion rings and toss to coat. Cover and let stand at least 30 minutes and up to 4 hours. Drain, reserving vinegar mixture.
Cook noodles in large pot of boiling water until tender but still firm to bite, about 2 minutes. Drain. Rinse under cold water. Drain well.
Whisk lime juice, oil and crushed red pepper in small bowl to blend. Stir in mint and cilantro. Using kitchen shears, cut noodles into 3- to 4-inch-long sections. Place handful of noodles in large bowl. Drizzle with reserved vinegar mixture and toss to coat. Ad remaining noodles, cucumber, green onions, tomatoes and marinated onions. Drizzle lime juice mixture over and toss gently. Season to taste with salt and pepper. Sprinkle with peanuts and serve.