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.