Brush the cut side of each leek half with the bacon drippings and sprinkle with kosher salt. Grill over direct, high heat, cut-side down, with lid closed, until grill marks appear, 6 to 7 minutes. Remove the leeks to a sheet of aluminum foil and lay cut-side up. Brush the leeks with balsamic vinegar. Reassemble the leek halves together, wrap tightly in foil, and set back on the grill away from direct heat for 10 to 12 minutes.