Bring a large pot of water to a boil. Add the lo mein noodles and cook according to package directions until al dente. Drain the noodles in a colander and rinse them with cold water. Return them to the pot and set aside.
Make the lo mein sauce:
Stir all ingredients together in a small bowl. Set aside.
Make the pan-fried tofu:
Coat the bottom of a large skillet with oil and place it over medium heat. When the oil is hot, add the tofu cubes. Cook them for about 10 minutes, flipping once or twice, until the cubes are browned on multiple sides. Remove the tofu from the skillet and transfer it to a plate.
Make the vegetable lo mein:
Using the same skillet that you cooked the tofu in, over medium heat, add 2 tablespoons of oil and arrange the shiitakes in an even layer. Cook until the mushrooms just begin to brown on the bottoms, about 3 minutes. Add the white parts of the scallions, garlic and ginger. Sauté for 1 minute.
Raise the heat to high. Add the bell pepper, snow peas and carrots. Stir-fry until the vegetables are just tender-crisp, about 1 minute. Add the noodles, sauce, tofu and green parts of scallions. Flip ingredients with a spatula to mix and coat noodles with sauce. Cook until the ingredients are well-mixed and the noodles are heated through, about 1 minute. Remove from heat.
Divide onto plates and serve.
If you want to keep this vegan, make sure to check the ingredients on your lo mein noodle package. Roland and Ka-Me brands are both vegan and available in supermarkets. If you can't find vegan lo mein noodles, spaghetti works as a substitute. Just boil pasta until fully cooked (not al dente), since spaghetti tends to be a bit firmer than lo mein noodles.