Whisk the sauce: in a medium bowl combine cornstarch, rice vinegar, soy sauce, sugar, 1/4 teaspoon pepper, minced garlic, and grated ginger until smooth.
Reserve 1/4 cup of the sauce in the refrigerator and pour the rest into a large freezer bag with the chicken; seal and marinate in the fridge 30 minutes to overnight.
Bring a pot of generously salted water to a boil and cook the vermicelli or thin spaghetti until al dente; drain and set aside.
Dry-roast the peanuts in a large wok or skillet over medium-high heat, tossing for about 3 minutes until golden; transfer to a plate.
Heat 1 tablespoon sesame oil in the wok or a large nonstick skillet over high heat. Add the marinated chicken (let excess marinade drip off) and stir-fry 3–5 minutes until cooked through; remove chicken to a plate.
Reduce heat to medium and add the remaining tablespoon of sesame oil. Add red pepper flakes or dried chilies, then sauté with garlic and ginger for about 30 seconds until fragrant.
Add the drained water chestnuts, chopped celery, chopped red bell pepper, and snow peas; sauté about 1 minute until vegetables are crisp-tender.
Return the cooked chicken to the wok, pour in the reserved 1/4 cup sauce, and bring to a boil; simmer about 30 seconds until the sauce thickens and coats the ingredients.
Stir in the roasted peanuts, chopped green onions, and cooked noodles; toss until everything is evenly combined. Taste and adjust salt, pepper, or extra red pepper flakes as desired.