Heat 2 to 3 tablespoons olive oil in a large skillet over medium-high heat.
Add the sliced chicken and cook for about 3 minutes, stirring and flipping occasionally, until lightly browned and cooked through; remove the chicken to a plate and reserve the pan juices.
Add a little more oil to the skillet if needed, then add the sliced Vidalia onion and sauté over medium heat for 5 to 7 minutes, stirring occasionally, until softened and beginning to caramelize.
Add the sliced red and yellow bell peppers and sauté about 5 minutes, stirring occasionally, until they begin to soften.
Return the cooked chicken to the skillet, add 1/4 cup water and the 1-ounce fajita seasoning packet, and cook about 3 minutes more, stirring occasionally, until heated through and the sauce is slightly thickened.
Warm the flour tortillas if desired (15–30 seconds per side in a dry skillet), spoon the fajita mixture into tortillas, and serve immediately with optional toppings.