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ROAST PORK:
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Preheat oven to 350u00b0F with rack in middle.
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Pat pork dry and season with 1 3/4 tsp salt and 1 1/2 tsp pepper. Straddle a flameproof roasting pan over 2 burners, then heat 1 Tbsp oil over medium-high heat until it shimmers. Brown pork on all sides, then transfer to a large plate.
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Put a metal rack in pan and arrange half of herbs down middle of rack. Stir together shallots, garlic, mustard, and 1 Tbsp oil and smear over top and sides of roast, then put roast, fat side up, on top of herbs. Roast 1 hour. Toss remaining herbs with remaining tsp oil and arrange on top of roast.
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Continue roasting until an instant-read thermometer registers 140 to 145u00b0F, 5 to 15 minutes more (temperature will rise 5 to 10 degrees as it rests). Transfer pork to a cutting board and let rest 15 to 25 minutes.
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MAKE SAUCE WHILE PORK RESTS:
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Remove rack from pan and discard herbs from rack. Straddle pan across 2 burners on medium heat. Add vermouth and mustard and deglaze by boiling, stirring and scraping up brown bits, until reduced by half. Add broth and simmer 3 minutes. Strain through a fine-mesh sieve into a 2-cup measure. If you have more than 1 1/2 cups, boil to reduce; if less, add water.
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Melt butter in a heavy medium saucepan over medium heat. Whisk in flour and cook, whisking, until pale golden, about 3 minutes. Whisk in vermouth mixture and simmer until slightly thickened, about 3 minutes.
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Serve pork with sauce.
