Cut the tenderloin on a slight angle into eight 1 1/4 inch medallions. Pound them lightly with the flat side of a large knife to flatten. On a plate, combine the flour with the salt and pepper. Dredge the medallions in the seasoned flour and shake off the excess. In a large nonreactive skillet, melt the butter in the oil over moderately high heat. Add the medallions in 2 batches and cook until nicely browned and medium rare, 2 to 3 minutes per side. Transfer to a plate, cover with foil and keep warm. Drain the fat from the pan. Add the vinegar and boil, scraping the bottom of the pan with a wooden spoon to release any brown bits, until the mixture is thick and reduce by about half. Add the chicken stock and any juices that have accumulated on the plate from the pork and boil until reduced to a dark, shiny sauce, about 1 minute. Add the sage and season to taste with salt and pepper. Arrange the medallions on a platter or plates and spoon the sauce on top.