Put the watermelon, tomatoes, cucumber, feta, olives, and dill in a large bowl. Drizzle with half the vinaigrette and toss gently, taking care not to break up the fruit and vegetables. Add more dressing if desired and toss again.
This salad can be made the night before you serve it and refrigerated, but in that case, don’t add the watermelon until ready to serve; refrigerate the watermelon separately. Let the salad and watermelon come to room temperature and toss together just before serving.
Put the vinegar, garlic powder, onion powder, oregano and za’atar in a small bowl. Whisk in the olive oil and season to taste with salt and pepper. The vinaigrette can be refrigerated in an airtight container for up to 3 days.