Pound chicken breasts to 1/4 inch thickness between two pieces of saran wrap. Whisk together first 7 ingredients. Place chicken in a large ziplock bag and pour marinade in and seal. Refrigerate at least 2 hours.
Remove chicken from marinade; discard marinade. Grill, covered with grill lid, over medium heat (300° to 350°) about 15 minutes on each side or until chicken is done. Cut into strips.
While the chicken is cooking, heat vegetable oil and butter in a large skillet on medium high heat. Add pepper and onion, sprinkle with garlic and onion powders, and sauté until tender.
Place chicken, pepper and onion on a hot platter. Serve in tortillas with desired toppings such as shredded cheese, sour cream, avocado, salsa, and lettuce.