The key word is 'convex'. If you imagine shrinking a rubber balloon over the vertices, any vertices which are not 'on the edge' will not touch the balloon. For a vertex to be part of the final shape, it must 'stick out'. If there's a vertex you need to have in the final shape, move it outwards, do the convex hull, and then move the vertex back.
It's more like putting a (deflated) balloon around the vertices, rather than 'vacuum packing'.
