1. Make glaze: Combine vinegar, soy sauce, olive oil, sugar and thyme in a small saucepan; season with salt and pepper. Bring to a boil over medium-high heat. Cook, stirring often, until reduced and slightly thickened, 7 to 10 minutes.
2. Make kebabs: Leaving some space between items and alternating, thread steak, tomatoes, zucchini and squash on 8 large or 16 small metal skewers. Place skewers on a large, rimmed baking sheet and season generously on all sides with salt and pepper. Brush liberally with glaze. (Discard any leftover glaze.) Cover skewers and refrigerate for up to 8 hours or overnight.
3. Bring kebabs to room temperature. Preheat gas grill on high heat for 10 to 15 minutes with lid closed. Oil grill. Place kebabs on grill, reduce heat to medium-high and close lid. Grill for 5 minutes, then turn kebabs and grill for another 5 minutes with lid closed. Open lid and check that vegetables are slightly charred and meat is medium-rare. If kebabs are not done, close lid and continue cooking until meat is cooked through. Transfer to a platter to serve.