There's a way to dual boot with the VHD method WITHOUT the use of external media.
This option is best for any individuals who wish to dual boot with Windows 10, but do not have access to any USB or DVD media.
The following is a way to set up a Windows 10 Technical Preview dual boot installation using a VHD/VHDX file, starting with a base OS of Windows 7/8/8.1:
It turns out this guide works for installing the Windows 10 Preview from within Windows 8.1, and there are no guides on the internet yet for this exact scenario!