1. Prepare a grill for medium heat (350° to 450°; you can hold your hand 5 in. above cooking grate only 5 to 7 seconds). Whisk together 1/2 cup cider, the vinegar, oil, shallot, salt, and 1 tbsp. sage in a large bowl. Add pork pieces to bowl and set aside.
2. When grill is hot, remove pork, reserving marinade, and cook, turning occasionally, until an instant-read thermometer inserted in center of meat reads 150°, about 20 minutes total. Transfer to a cutting board and tent with foil to rest.
3. Meanwhile, in a small saucepan, bring marinade and remaining 1/2 cup cider to a boil. Reduce heat and simmer until liquid is reduced by half, 6 to 7 minutes. During last minute, whisk in butter.
4. Cut pork into 1/4-in. slices, drizzle portions with sauce, and sprinkle with remaining sage.
Note: Nutritional analysis is per serving.