Cut peppers into thin strips; then, in a medium-sized skillet, saute over high heat in 2 tablespoons of the oil until peppers are browned in spots. Set aside. Cut corn from the cob and dice potatoes fairly small. Mix remaining ⅓ cup of oil with onion, herbs, mustard, soy sauce, and vinegar. Stir in the peppers, then add the remaining vegetables and season to taste. Stir well and chill in a tightly covered 2-quart bowl for at least 2 hours, so flavors have a chance to blend. This salad will keep well in the refrigerator for 2 or 3 days.