In the bowl of a food processor, combine vinegar, oil, thyme, rosemary, oregano, garlic, salt, and freshly ground pepper. Pulse until the garlic is very finely chopped and you have one cohesive mixture.
Take a sharp knife or a fork and prick the lamb all over on both sides. Place the lamb in a large dish and pour the marinade over the meat. Leave in the fridge overnight, up to 24 hours.
When you are ready to grill, pull the lamb out and let it come to room temperature. Then, get your grill ready. Leg of lamb is best grilled at a medium temperature over indirect heat. This is easy to achieve with a gas grill, but if you’re using charcoal, bed your charcoal over to one side and let it burn for about 10-15 minutes before you get grilling on the side with no charcoal.
Grill the lamb for about 15 minutes per side, or until you reach the desired level of doneness.