So I saw today that Mark Russinovich have released a new version of the Disk2VHD binary from SysInternals

One really nice new feature is that you can with this updated tool now create VHDX directly without needing to convert a VHD and that does not require a Windows 2012 or Windows 8 or later to be able to work. This tool can be used when doing P2V also.

As you can see on my screendump I have tested it on a Windows 7 and the checkbox with VHDX.

You can find it here on SysInternals page http://technet.microsoft.com/en-us/sysinternals/ee656415