From what I remember about the directions that came with my vice the three holes that hold the head to the arm are for adjusting the jaws to hold diifferent size hooks one hole for 3/0 - ?? another for "medium" size hooks and the other for the small ones ??-28. To make the hook line up on the center line you'll need to change the point at which you attach the hook in the jaws. If you are currently placing the jaws someplace behind the barb on a size 14 and it's centered, you'll need to place the jaws higher up on the bend if you switch to a size 10 to keep it centered.

Kevin