Position a rack in the lower third of the oven and preheat to 450 F (230 C). Lightly coat a baking sheet with olive oil cooking spray.
To make the vinaigrette, whisk together the vinegar, lemon juice, oregano, salt and pepper in a small bowl. While whisking, slowly add the olive oil in a thin stream until emulsified. Set aside.
Spread the eggplant cubes in a single layer on the prepared baking sheet. Spray the eggplant with olive oil cooking spray. Roast for 10 minutes. Turn the cubes and roast until softened and lightly golden, 8 to 10 minutes longer. Set aside and let cool completely.
In a large bowl, combine the spinach, cucumber, tomato, onion and cooled eggplant. Pour the vinaigrette over the salad and toss gently to mix well and coat evenly. Divide the salad among individual plates. Sprinkle with the olives and feta. Serve immediately.