1. Preheat grill to medium-high. In a bowl, toss tomatoes, zucchini, eggplant, bell pepper, garlic, thyme, coriander and crushed red pepper, if desired, with 1 Tbsp. oil. Season with 1/2 tsp. salt and 1/4 tsp. pepper. Place mixture on a large rectangle of heavy-duty foil. Gather edges; fold to form a well-sealed long rectangular packet.
2. Season chicken with 1/4 tsp. each salt and pepper. Toss with 1 Tbsp. oil. Brush polenta with remaining 1 Tbsp. oil.
3. Oil grill grates. Place vegetable packet, chicken and polenta on grill and close cover. Grill packet, shaking occasionally and flipping once, until contents are sizzling and eggplant is soft, about 15 minutes. Grill chicken, turning once, until lightly charred on outside and cooked through, 10 to 15 minutes. Grill polenta, turning once, until light golden and grill marks appear and it releases easily from grates, 6 to 8 minutes per side. Transfer polenta, chicken and vegetables to platter as they are finished cooking; serve.