Prepare the tofu by pressing it to remove excess water, then cut it into small cubes. In a bowl, mix together the soy sauce, oyster sauce, sesame oil, rice vinegar, sugar, red pepper flakes (if using), water, and cornstarch. Add the cubed tofu to the bowl and marinate for at least 10 minutes.
Cook the rice stick noodles according to package instructions, usually by soaking in hot water for about 5 minutes until softened. Drain and set aside.
While the noodles are soaking, prepare the vegetables. Thinly slice the Chinese cabbage, julienne the carrot, and thinly slice the red bell pepper. Mince the garlic and grate the ginger.
Heat the vegetable oil in a large wok or deep frying pan over medium heat. Add the marinated tofu and cook until browned on all sides. Remove the tofu from the pan and set aside.
In the same pan, add a bit more oil if needed, then add the garlic and ginger. Stir-fry for about 30 seconds until fragrant.
Add the sliced vegetables (cabbage, carrot, and bell pepper) to the pan and stir-fry for about 3-4 minutes until slightly softened but still crisp.
Add the cooked rice stick noodles and the cooked tofu back into the pan. Pour any remaining marinade over the noodles and vegetables. Stir-fry for another 2-3 minutes until everything is well combined and heated through.
Season with salt and pepper to taste.
Garnish with chopped green onions and sesame seeds before serving.