Combine soy sauce, vinegar, pepper, sliced onion, and 1 cup water in a large bowl; add chicken and cover bowl with plastic wrap. Refrigerate for at least 8 hours or overnight to marinate.
Build a medium-hot fire in a charcoal grill or heat a gas grill to medium-high. (Alternatively, heat a cast iron grill pan over medium-high heat.) Cook chicken, turning once, until just cooked through. Transfer to a cutting board and let cool; cut into 1/2" cubes and place in a large bowl. Add chopped onion, scallions, coconut, juice, and chiles, and toss to combine; let sit at room temperature for 1 hour to meld flavors. Serve with pita, if you like.