In a medium bowl, combine the diced chicken, 1 tablespoon of soy sauce, rice vinegar, and cornstarch. Mix well and let marinate for 15 minutes.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the marinated chicken and cook until browned and cooked through, about 5-7 minutes. Remove the chicken from the skillet and set aside.
In the same skillet, add the remaining tablespoon of vegetable oil. Add the red bell pepper, snap peas, garlic, and ginger. Stir-fry for about 3-4 minutes until the vegetables are tender-crisp.
Return the cooked chicken to the skillet. Add the cooked egg noodles, remaining soy sauce, sesame oil, and crushed red pepper flakes (if using). Toss everything together and cook for an additional 2-3 minutes until heated through.
Stir in the roasted peanuts and green onions just before serving.