1. Preheat oven to 450 degrees F.
2. In a food processor, combine rosemary leaves, garlic, oil, vinegar, salt, and pepper and pulse to make a coarse, wet paste, scraping down the bowl as necessary. Spread 3/4 of paste on all sides of roast.
3. Place roast, fat side up, in shallow roasting pan large enough to hold roast with 3 inches room around all sides. Roast 15 minutes.
4. Meanwhile, in a large bowl, combine potatoes and remaining paste.
5. Reduce oven to 350 degrees F.
6. Add potato mixture to pan, arranging potatoes around roast. Continue roasting 40-45 minutes, tossing potatoes halfway through, or until internal temperature of roast reaches 145 degrees F and potatoes are tender. (If roast is done before potatoes, transfer it to a cutting board and return-roasting pan with potatoes to oven.)
7. Remove roast from oven and let rest 10 minutes. Slice roast and arrange on platter. Surround with potatoes and serve.