Prepare the sauce by whisking together dark soy sauce, water, sesame oil, vegan oyster sauce, rice vinegar, maple syrup, cornstarch, sriracha, and vegetable bouillon in a small bowl. Set aside.
Bring a large pot of water to a boil and cook the chow mein noodles according to the package directions until just tender. Avoid overcooking because the noodles will continue cooking during the stir-frying process.
Drain the noodles and rinse briefly under cold water to stop the cooking. Toss with a small drizzle of oil if needed to prevent sticking.
Heat 1 tablespoon of vegetable oil in a large wok or skillet over medium-high heat. Add the tofu slices or vegan chicken pieces and cook for 5–7 minutes until golden and lightly crisp. Transfer to a plate and set aside.
Add the remaining tablespoon of oil to the pan. Add the onion and garlic and stir-fry for 1–2 minutes until fragrant and slightly softened.
Add the carrots, cabbage, bell pepper, and bok choy stems. Cook for 3–4 minutes, stirring frequently, until the vegetables are tender but still slightly crisp.
Return the cooked tofu or vegan chicken to the pan along with the bok choy leaves. Stir everything together until warmed through.
Add the cooked noodles and pour the prepared chow mein sauce over the top. Toss continuously until the noodles are evenly coated and the sauce thickens slightly.
Continue cooking for another 2–3 minutes, allowing some noodles to lightly crisp against the pan for extra texture.
Remove from heat and garnish with green onions and sesame seeds. Serve immediately.