Whisk together rice vinegar, fish sauce, lime juice, palm sugar, red pepper flakes, and black pepper in a large bowl.
Marinate chicken into rice vinegar mixture; set aside while preparing remaining ingredients.
Separate rice noodles in warm water, about 5 minutes; drain.
Heat the vegetable oil in a large skillet or wok over medium heat. Cook and stir the garlic and onion in the hot oil until the onion is soft and translucent, about 5 minutes.
Increase heat to medium-high. Stir chicken and marinade into onion mixture; cook and stir until chicken is no longer pink in the center and sauce begins to thicken, 7 to 10 minutes.
Stir rice noodles, oyster sauce, soy sauce, red bell pepper, tomatoes, green onion, and basil leaves into the chicken mixture; cook and stir until sauce is evenly distributed and noodles are tender, 5 minutes more.
