1. Rinse pork and pat dry. Stir together vinegar, garlic, and pepper in a cup. Place in a large heavy-duty resealable plastic bag with pork and seal, turning until coated well. Marinate at room temperature, turning bag occasionally, 30 minutes.
2. Preheat oven to 450 degree F. Arrange rack in middle of oven. Lightly oil a baking sheet. Cut off dark-green tops from leeks and reserve for another use. Halve, lengthwise, the remaining white and pale-green part of leek and clean (see cue). Arrange asparagus and leeks in a single layer on prepared baking sheet. Toss with oil; season with salt and pepper.
3. Remove pork from marinade and place on vegetables. Pour marinade over pork; season with salt. Roast, turning pork and vegetables halfway through, until an instant-read thermometer inserted in center of pork registers 150 degree F and vegetables are tender, about 30 minutes.
4. Let pork stand in pan 10 minutes before carving into 1/4-inch-thick slices. Arrange pork slices over vegetables on a serving platter. Makes 4 servings.