Coat the bottom of a skillet with vegetable oil and place over medium heat. Saute chicken in the hot oil until no longer pink in the center, 5 to 10 minutes. Add rice vinegar, soy sauce, mustard, chile oil, and Sriracha sauce, adding in exact order and waiting 1 minute between each addition for ingredient to layer.
Melt butter, stirring occasionally, into the chicken mixture creating a sauce, 2 to 3 minutes; lower heat to medium-low. Add green onions and cook until slightly wilted, 2 to 3 minutes.