Disk2vhd will ony create one VHD for each separate disk on which selected volumes/partitions reside on.
For example, my C:\ Windows 7
drive (partition) and System Reserved
(partition) are both on the same physical Disk 2 as seen in Disk Management (diskmgmt.msc). I will only have one VHD file created for these two. I would get separate VHD file(s) created if I were to also select my D:\ Data Backup
and/or E:\ System Backup
drives since they are both on separate physical disks.