My overall goal is to take an about-full VHD and resize it so more data can be put into it.
I know ResizeVirtualDisk() is available only since Windows 8. That's not my problem. The problem is that I can't seem to find any way to resize the volume to fill the resized VHD. The closest I could find was IVdsVolume::Extend(), but apparently all this does is span a volume across disks. So now I'm completely stumped.