Vine pieces are whole pieces of the vine, not hammered into small pieces. The shredded is the same plant material, but shredded into small pieces. Isn't this kinda obvious? Some people claim that dealing with whole vine conserves more of the potency of the plant, and they may be correct, especially if it's stored for a long time.
However, I personally enjoy using shredded vine, as you then don't need to break the vine into small pieces personally. Others would argue that shredding ones vine is an important step in the preparation, which may be seen upon as a part of the experience itself. I'd say that's far from wrong.