Heat the vegetable oil in a large skillet over medium heat.
Add the diced onions and bell peppers, sautéing until soft.
Stir in minced garlic and cook for another minute until fragrant.
Add the ground chicken to the skillet, breaking it apart with a spatula.
Cook the chicken until it's no longer pink, about 5-7 minutes.
Stir in the soy sauce, hoisin sauce, rice vinegar, salt, and pepper.
Cook for an additional 3-4 minutes until everything is heated through.
Remove from heat and garnish with chopped green onions.
Serve the chicken mixture in lettuce leaves.