Preheat a grill pan over high heat. Season chicken with Italian seasoning and place on grill. Cook, basting with balsamic vinaigrette and turning once, until cooked through, about 2 minutes per side. Rremove from grill; set aside.
Whisk together the red wine vinegar, olive oil, and dried oregano.
Place shredded romaine lettuce and red onions in a medium bowl; drizzle with red-wine vinaigrette and toss to combine.
Combine all the tzatziki ingredients together. Divide lettuce & onion mixture evenly amount butter lettuce leaves and drizzle each with about 1 teaspoon or tzatziki.
Top each taco with 1 piece of chicken and garnish with chopped tomatoes, feta cheese, and olives. Garnish with parsley; serve.