In a small bowl, stir together teriyaki sauce, vinegar, ginger, sugar and garlic. Reserve 3 tablespoons for the noodles. Marinate tuna in the remaining teriyaki sauce mixture for at least 15 minutes.
Prepare soba noodles following package directions, adding snap peas during the last 3 minutes of cooking. Drain, rinse and place in a medium-sized bowl. Toss with red pepper slices and reserved teriyaki sauce mixture. Let stand at room temperature.
Coat a grill pan with non-stick cooking spray and heat over medium-high heat. Grill tuna for 3 minutes. Turn and grill for an additional 2 minutes for medium doneness.
Serve with soba noodles and garnish with scallions. Toss noodles with additional teriyaki sauce, if desired.