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Grind 1/4 cup pink peppercorns with black peppercorns and fennel seeds in grinder, then stir together with garlic, oil, and 1 Tbsp salt.
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Pat pork dry and use a paring knife to make about 16 (1-inch-deep) slits all over roast. Stuff slits with all but 1 Tbsp garlic-peppercorn paste, then rub remaining Tbsp all over roast. Put in a small (13- by 9-inch) flameproof roasting pan and marinate, chilled, 8 to 24 hours.
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Let pork stand at room temperature 1 hour. Preheat oven to 350°F with rack in middle.
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Roast pork, fat side up, until an instant-read thermometer inserted into center of meat registers 150°F, 1 1/2 to 2 hours. Transfer pork to a cutting board and let rest 30 minutes.
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Meanwhile, pour off all but about 1 Tbsp fat from roasting pan. Add vermouth to pan and boil, scraping up brown bits, 2 minutes. Stir in broth, any meat juices from cutting board, and remaining 2 Tbsp pink peppercorns and boil until reduced to about 11/2 cups, about 5 minutes.
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Knead together butter and flour, then whisk into sauce and boil, whisking constantly, until just slightly thickened, about 2 minutes.
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Serve pork with sauce.
