Cook the noodles according to package instructions. Drain and set aside.
In a small bowl, mix soy sauce, hoisin sauce, rice vinegar, brown sugar, and sesame oil. Set aside.
Heat vegetable oil in a large skillet or wok over medium-high heat.
Add the chicken slices and cook until browned and cooked through, about 5-6 minutes.
Stir in garlic, ginger, and red pepper flakes. Cook for 1 minute.
Add bell pepper and cook for another 2-3 minutes until tender-crisp.
Pour the sauce over the chicken and vegetables, stirring well to coat.
Add the cooked noodles and toss everything together.
Stir in peanuts and green onions just before serving.