You can avoid using unintended verts by creating a new group consisting of only the materials you want the magnet to effect. In this case you would only choose the entire neck materials, then create another group on the head, using only the head skin mats. When you include the head in the magnet properties as an additional element to deform (assuming the magnet is parented to the neck), check the grouping box in the properties window of the magnet zone and check the appropriate group (head skin). the magnet will then effect the neck and the head, but not the eyes or any part of the mouth.