In a zip-top plastic bag, combine chicken and vinaigrette. Refrigerate 1 hour (or up to 12 hours). Preheat grill. In large bowl, toss zucchini, tomatoes, and precooked potatoes with oil and season with salt and pepper. Put vegetables in grill wok or grill basket and grill 10 minutes. Add chicken to grill and cook until chicken is done and vegetables are tender, about another 5-10 minutes.
Transfer chicken to a cutting board and let rest. Toss vegetables with spinach and divide among plates. Slice chicken. Serve chicken on top of salad, with lemon wedges.
Why are the potatoes turning into charcoal before the zucchini even softens? Parcooking starchy tubers is essential here, meaning the Yukon Gold potatoes must be boiled in salted water until a paring knife slides halfway through with moderate resistance before they ever touch the grill grates. Without this 10-to-12-minute head start, dense potato cubes remain chalky in the center long after thin summer squash moons have burned. Marinating poultry in an acidic vinaigrette breaks down exterior proteins, but letting chicken breasts soak longer than 12 hours converts tender meat into a mealy texture as the vinegar denatures muscle fibers past the point of recovery. Utilizing a perforated grill wok or a heavy-gauge mesh basket prevents small cherry tomatoes and potato cubes from slipping through open bars into the flames while still allowing direct contact with smoke. Tossing raw baby spinach with warm, freshly grilled vegetables immediately wilts the greens into a silky bed without requiring a separate pan or steaming step. High heat is mandatory for proper charring on poultry, requiring a grill preheated to 400 degrees Fahrenheit before the meat goes on. Resting cooked chicken for at least 5 minutes on a clean cutting board allows internal juices to redistribute throughout the muscle fibers instead of pooling across the plate upon slicing. Combining oil and dry seasonings with the vegetables in a separate mixing bowl guarantees even coating before cooking, whereas dumping oil directly onto produce in a hot basket results in patchy spots and flare-ups. Budget-conscious cooks can easily substitute zucchini with yellow crookneck squash at a one-to-one ratio without altering cooking times. Leftover grilled chicken and vegetables store safely in an airtight container in the refrigerator for up to 3 days, though the spinach should be added fresh upon reheating to maintain structural integrity.