Put 1/2 cup olive oil and garlic in a small saucepan and turn heat to high. When garlic begins to turn light brown, add the peppers and cook until garlic is medium brown. Turn off the heat and cool to room temperature. Then add the remaining 1/4 cup olive oil, vinegar, scallions, and parsley.
Preheat a grill.
Wrap each shrimp with a piece of pancetta. Skewer a couple onto the woody end of each rosemary sprig. Grill the shrimp for 2 minutes on each side. Keep the rosemary over the end of the grill away from the direct flame. Transfer to a serving platter and spoon over vinaigrette.
Chef's note: If preparing this dressing 1 day ahead, add the scallions and parsley just before serving.