To make Teppanyaki sauce:
Boil balsamic vinegar in a large, wide frying pan over high until reduced by half, 2 to 3 min. Add remaining ingredients to pan. Continue boiling, whisking often, until slightly thickened and reduced to 1 cup, 2 to 4 more min. Cool completely.
To make Grilled tofu teppanyaki:
Soak skewers in water for 20 min. Pat 340 g firm tofu dry with paper towels. Cut into large cubes and marinate with 1/2 cup Teppanyaki Sauce in a freezer bag. Refrigerate for 4 hours or overnight. Preheat barbecue to medium. Thread tofu alternately with yellow zucchini and red onion chunks onto skewers. Oil grill and barbecue skewers until tofu is hot and veggies are cooked, 2 to 3 min per side.