In a ziploc bag, combine chicken, zucchini, onion, olive oil, oregano, and 2 Tbsp vinegar. Marinate at room temperature for 30 minutes.

Sauce: Blend feta, yogurt, mint and remaining vinegar in food processor until smooth.

Heat grill to medium. Lightly oil grates. If using wooden skewers, soak them in water for 15 minutes. Thread 3 pieces of chicken with zucchini and onion on skewer. Grill skewers, turning occasionally, until chicken is cooked through and vegetables are tender, 12-15 minutes.

Serve with sauce for dipping.