In a large bowl, combine the diced chicken with 2 tablespoons soy sauce, rice vinegar, and cornstarch. Stir to coat evenly and set aside for 10 minutes.
Cook udon noodles according to package directions, drain, and set aside.
In a small bowl, add the remaining 2 tablespoons soy sauce, oyster sauce, toasted sesame oil, garlic, and ginger. Stir well to combine and set aside.
In a large skillet, add 1-2 tablespoons of neutral oil, the marinated chicken, and cook over medium-high heat for about 3 minutes, or until done. Remove from the skillet and set aside on a plate.
To the same skillet, add more oil if necessary, add the onions, carrots, and cook for about 2-3 minutes, or until they're becoming softer.
Add the bok choy and the white parts of the scallions, and cook for 2-3 minutes more, stirring frequently.
Add the cooked noodles and pour the stir fry sauce over everything. Toss to combine and coat evenly.
Add the cooked chicken, stir and toss to combine with the noodles, and stir fry for 1 minute, or until warmed through.
Taste the stir fry and if desired, add additional soy sauce, oyster sauce, and garnish with red chiles and sesame seeds.
