Combine first 4 ingredients in a small saucepan over high heat; bring to a boil. Reduce heat to medium; gently simmer, stirring occasionally, 6 minutes or until reduced to about 1/3 cup. Pour reduction into a small bowl; set aside.
Process garlic and 2 tablespoons olive oil in a food processor until smooth, scraping sides of bowl. Set aside.
Preheat grill to medium-high heat (350° to 400°). Place asparagus, zucchini, and squash in a large bowl. Drizzle with 2 tablespoons olive oil, and sprinkle with 1/4 teaspoon each salt and pepper; toss gently to coat. Grill 4 minutes or until crisp-tender, turning after 2 minutes. Keep warm.
Sprinkle semolina flour evenly over a large, rimless baking sheet. Place pizza dough on baking sheet, and pat into a 14-inch circle.
Brush grill grates with oil, and slide pizza dough onto grates. Brush top of dough with remaining 1 tablespoon oil. Grill 2 minutes or until grill marks appear. Reduce heat to medium-low; flip dough, and spread evenly with garlic mixture, leaving a 3/4-inch border. Top with grilled vegetables, and sprinkle evenly with cheese. Grill, covered with grill lid, 5 to 7 minutes or until cheese melts. Remove pizza from grill, and drizzle with desired amount of balsamic reduction. Serve immediately.