In a large plastic bag, mix together the vinegar, olive oil, and rosemary.
Add the racks of lamb and allow them to marinate in the refrigerator overnight.
Preheat the oven to 450 degrees.
Remove the lambs from the bag and rub them down with the salt and pepper. Then roll them in the crushed pecans. If they are having a hard time sticking, put some more of the marinade on them to give the pecans something to cling to.
Place the racks of lab in a roasting pan and cover the tips of the bones in tin foil so that they do not char while cooking.
Cook for 20 minutes, then lower the heat to 400 and cook for another 10 to 15 minutes, or until the lamb has an internal temperature of at least 145 degrees Fahrenheit.