Preheat oven to 450degrees F. In food processor, combine rosemary, garlic, oil, vinegar, salt and pepper; pulse to a coarse, wet paste.
Spread 3/4 paste on all sides of roast. Place roast, fat side up, in shallow roasting pan big enough to hold roast with 3" around all sides. Cook 15 minutes.
Meanwhile, in large bowl, mix potatoes and remaining paste. Reduce heat to 350 degrees F. Place coated potatoes around roast; cook roast until internal temp. Reaches 150degrees F. If potatoes aren't done, transfer roast to cutting board and continue cooking potatoes.
Remove roast from oven, let rest 10 minutes. Slice and arrange on platter with potatoes.