Toast juniper berries in a small skillet over medium-high, swirling, until fragrant, 5 minutes. Transfer to a food processor and blend with verjus, 1⁄2 cup salt, the sugar, parsley, marjoram, and 1⁄2 cup water until smooth. Pour into a large bowl and add pork chops. Cover and refrigerate at least 12 hours or overnight.
Meanwhile, marinate the leeks: Stir remaining 2 tbsp. salt, olive oil and vinegar in a bowl until emulsified. Add leeks, cover with plastic wrap, and marinate for 2 hours.
Drain pork chops and set aside for 30 minutes to come to room temperature; reserve the marinade. Build a medium-heat fire in a charcoal grill, or heat a gas grill to medium. (Alternatively, heat a cast-iron grill pan over medium.) Grill the pork chops, turning and basting every 5 minutes with reserved marinade, until a thermometer inserted into the thickest part of the meat reads 140°, 40–45 minutes. Transfer the pork chops to a cutting board and let rest for 10 minutes.
Meanwhile, remove leeks from vinaigrette and arrange on a sheet of foil. Pour 2 tbsp. vinaigrette over leeks, and then fold and close foil into a packet; reserve remaining vinaigrette. Place packet on grill and cook until leeks are soft and caramelized, 8–10 minutes. Open packet and transfer leeks to a serving platter; discard foil. Drizzle leeks with some of the remaining vinaigrette and season with pepper. Slice pork chops and serve with the grilled leeks.