Chop red onion, yellow pepper, red pepper, and orange pepper. The larger the better! Your veggies will shrink when grilling so typically about 2 inches x 2 inches works great! Place chopped veggies with mushrooms into a large mixing bowl.
Combine soy sauce, oil, white wine vinegar, sugar, oregano, & basil in a smaller mixing bowl and whisk together.
Pour the marinade over veggies in a larger bowl and cover with a lid. Gently, toss the vegetables in the marinade.
Place veggies and marinade in the refrigerator and let sit for 4-6 hours.
If using wooden skewers; it is important to soak the skewers just prior to grilling for about 20 min. (or follow the skewer package instructions).
After 4-6 hours, remove the veggies from the refrigerator and begin stacking the veggies onto the skewers. Interchange the pepper colours, onion, and mushrooms. I like to ensure that every skewer has 2 mushrooms at least; as they burst with flavor. (Alternately when in a rush I have omitted the skewers and just tossed the marinated veggies in a grill pain over the BBQ or on the stove..... the flavor is still there but the skewers are more fun!)
Preheat grill to medium heat (375-400 degrees Fahrenheit).
Oil the grill gates with a little bit of vegetable oil and silicone brush.
Grill for a total of approximately 4 min per side; or until veggies appear lightly charred and tender.