In a small bowl, whisk together soy sauce, rice vinegar, ginger, garlic powder, and red pepper flakes (if using). Set aside.
Heat olive or sesame oil in a large skillet or wok over medium-high heat. Add chicken strips and cook until browned and cooked through, about 5-7 minutes. Remove chicken from skillet and set aside.
Add broccoli florets to the skillet and stir-fry for 3-4 minutes until slightly tender-crisp.
Add red bell pepper, snap peas, and water chestnuts (if using) to the skillet. Stir-fry for another 2-3 minutes.
Return chicken to the skillet. Pour the prepared sauce over the chicken and vegetables, tossing to coat. Cook for 1-2 minutes until the sauce slightly thickens.
Serve hot over cooked brown rice or quinoa.
