Preheat a large skillet over medium-high heat. Add olive oil.
Add sliced bell peppers and onion to the skillet. Cook until softened, about 5-7 minutes.
Add chili powder, cumin, paprika, garlic powder, onion powder, salt, and pepper. Stir to coat the vegetables evenly.
Warm the tortillas in a separate pan or microwave.
Assemble the tacos by placing the cooked vegetables on the tortillas, drizzling with vegan ranch dressing, and topping with avocado slices and fresh cilantro.