In my understanding an "hdisk" is a single istance of a lun as seen by the host through the SAN (or, if you prefer, a single path through the SAN to reach a lun) while a "vpath" is the common name you use to reach the same lun using all the different hdisks (paths) through the SAN.

I would defer that to IBM for more clarity and understanding - since these types of paths are seen with their SDD installation packages. Check out their website for more information. But appears to be for their product.. I had found a link and hopefully is a helpful start for you:

http://www-01.ibm.com/support/docview.wss?uid=ssg1S7000303&aid=1