In the container of a blender or large food processor, combine the vegetable broth, raspberries, raspberry jam, and red wine vinegar. Puree until smooth.
Pour into a large bowl, and set aside.
Heat a nonstick skillet over medium-high heat. Sear the leg of lamb quickly on all sides, and remove from the pan. Pierce the meat every inch or so with the tines of a fork.
Place into a bowl with the raspberry sauce. Cover, and refrigerate for at least 12 hours to marinate. Turn occasionally.
Preheat the oven to 350 degrees F (175 degrees C).
Remove the lamb from the marinade and pat dry.
Place on a roasting rack in a roasting pan.
Roast the lamb for 1 hour in the preheated oven, or until a meat thermometer inserted into the meat reads 140 degrees F (62 degrees C). Allow to stand for 10 to 15 minutes before carving. The temperature should go up by another 5 to 10 degrees while it rests for medium rare. If you like it more done, wait until it reaches 145 degrees F before removing from the oven.
While the lamb is roasting, transfer the marinade to a saucepan. Bring to a boil, and add the rosemary. Boil over medium-high heat until the sauce has reduced and thickened slightly, about 10 minutes. Carve the lamb, and spoon sauce over it to serve.
