Mix the soy sauce, vinegar, white pepper, garlic powder, and salt in a bowl. Add the chicken and mix well, then add the baking soda and mix again. Rest for 15 minutes.
Meanwhile, boil the spaghetti in salted water until al dente. Drain.
Heat 1 tbsp oil in a wok, add the marinated chicken, and cook for 4–5 minutes until done. Remove from the pan.
In a small bowl, mix all the chowmein sauce ingredients until well combined.
Heat 1–2 tbsp oil, add the carrots, bell pepper, and cabbage, and sauté for 2 minutes. Set aside.
Heat 2–3 tbsp oil, add the sliced garlic, and cook for 1 minute. Add the spring onions and sauté for 1 minute.
Add the boiled spaghetti and toss. Add the cooked chicken and mix lightly.
Pour in the chowmein sauce and cook for 1 minute. Add the stir-fried vegetables and toss for 1 minute.
Add the green onion leaves, cook for 1 more minute, and serve hot.