Using just soy sauce instead of Teriyaki sauce is not a great idea, as soy sauce is much saltier and doesn't have the same sweetness as Teriyaki sauce. If you don't have Teriyaki sauce, either make your own in 10 minutes with this 3-ingredient Teriyaki sauce recipe or use a smaller amount of soy sauce (3 Tbsp) and 2 minced garlic cloves. Sauté the garlic for a minute before adding the veggies, and take care not to burn it. Then add the soy sauce when you would otherwise add the Teriyaki sauce.