Combine the tamari and rice vinegar in a small bowl, and set aside.
Season the chicken with salt and pepper. Heat oil in a large heavy skillet over high heat. Add garlic and ginger and stir until fragrant, about 30 seconds.
Add chicken to skillet and stir-fry until white, about 2 minutes. Add asparagus, bell pepper, onion and crushed red pepper, if desired, and stir-fry until vegetables are crisp-tender and chicken is just cooked through, about 3 minutes.
Add soy sauce mixture, stir, and cook for about 1 minute more. Serve immediately.