Heat oil in a medium, heavy-bottomed skillet over high heat. Pat pork dry, and season with salt and pepper. Cook pork in oil until brown, about 4 minutes or until cooked to desired doneness. Transfer to a warm platter; set aside.
Remove skillet from heat. Discard fat, and add port; return skillet to heat. Deglaze pan, scraping with a wooden spoon to loosen the browned bits from the bottom.. Continue cooking until sauce is reduced to about 2 tablespoons. Add stock, and cook until sauce is lightly syrupy, about 4 minutes. Whisk in butter, a little at a time, gently shaking pan while whisking. Stir in vinegar. Taste, and adjust for seasoning. Spoon sauce over pork, and serve immediately.
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