Cut zucchini diagonally into 1/2-inch-thick slices; cut onion into 1/4-inch-thick slices. Cut bell peppers into 1-inch-wide strips.
Stir together soy sauce and next 3 ingredients in a large bowl; add vegetables, tossing gently to coat. Let mixture stand at room temperature 1 hour, stirring occasionally.
Coat food rack with cooking spray; place on grill over medium-high heat (350° to 400°). Arrange marinated vegetables on rack, and grill, without grill lid, 6 to 8 minutes on each side.
Trim 1/2 inch from each end of focaccia, and cut bread into 4 equal squares. Slice each square horizontally in half. Toast bread on grill, if desired. Spread cut sides evenly with Herbed Goat Cheese; place grilled vegetables evenly on bottom halves of bread, and then top with remaining bread. Serve sandwiches immediately.