Cut lamb into 1-inch pieces. Mix oil, vinegar, garlic, chopped rosemary, and salt; add lamb and refrigerate for up to 8 hours. When ready to grill, bring lamb to room temperature. Cut onions and peppers into chunks and skewer, alternating with the lamb. When coals are hot, dampen extra rosemary sprigs and toss on fire. Grill skewers over coals, brushing with marinade and turning until done, about 8 to 10 minutes.