Scrub the whelks under cold running water. Transfer to a large saucepan and cover with cold water, the vinegar, cracked black pepper, and 1 tablespoon salt. Bring to a boil, cover, reduce the heat and, simmer, skimming as needed, for 1½ hours. Strain and, when cool enough to handle, pick the meat from the shells. Separately rinse the meat and shells with cold water. Return the shells to the saucepan, cover with water, and boil for 10 minutes. Trim the guts from the meat, leaving just the muscle. Rinse the muscle, pat dry, and toss with olive oil, salt, and white pepper to taste. Strain the shells and chill under cold running water. Return the whelks to the shells for presentation.