Place the artichokes on a steamer rack over boiling water, cover and steam until a fork easily pierces the base of an artichoke, 30 to 40 minutes. Remove from the pan and set upside down to drain and cool.
Meanwhile, put the bread in a bowl and add half of the vinegar and half of the water. Let soak, turning occasionally, until the bread absorbs the liquid to the extent that it crumbles easily between your fingertips. The drier and coarser the bread, the more liquid you will need, so add as much of the remaining vinegar and water as necessary. When enough liquid has been absorbed, crumble the bread into soft crumbs. Add the 1/4 cup olive oil, the tomatoes, minced parsley, capers, the 1 tsp. salt and the 1/2 tsp. pepper and mix well. The stuffing should be quite moist. If necessary, add a little more vinegar, oil or other seasonings to your taste.
To prepare each artichoke for stuffing, gently spread the inner leaves apart. Using a metal spoon, scoop out the small, pale inner leaves and the hairy choke to create a cavity. Fill the cavity with some of the stuffing, mounding it slightly. Then gently separate the outer leaves and, using a teaspoon and your fingertips, tuck in a little stuffing at the base of each row of leaves. Set the artichoke on a platter and repeat with the remaining artichokes.
Cover the platter with plastic wrap and refrigerate the artichokes for at least 4 hours or up to 12 hours before serving. Transfer to individual plates and serve chilled, garnished with parsley sprigs.