Place the cucumber, peppers, tomatoes zucchini, summer squash, and red onion in a large bowl.
To make the vinaigrette, combine together the vinegar, mustard, garlic, oregano, salt and pepper and whisk in a small bowl. Continue whisking while slowly adding the olive oil to emulsify the dressing. Pour the vinaigrette over the vegetables. Add the feta and olives and toss lightly. Set aside for 30 minutes to allow the flavors to blend. Serve at room temperature. This dish can be stored in the fridge for up to 3 days.