It is not supported!..
It looks like you need more then just
bootmgr for booting a vhd. And these components will only be available on Windows 7 Ultimate and Enterprise Plus Windows Server 2008 and 2008 R2. Plus i think it is enough with WIM support under
Windows does not support booting to a remote share or a USB flash drive, whether installed on a physical volume or from a VHD. You can boot the Windows Pre-installation Environment (Windows PE) from a USB flash drive, which is supported for Windows deployment. Windows PE typically boots from either a Boot.wim or an installed image, but booting Windows PE from a VHD is not supported.