Only hdd v2 is supported by VirtualBox, as the format is otherwise closed and proprietary.
If it was me and I really wanted to escape the Parallels net, I would run Disk2VHD inside the Parallels VM, writing the image to a network drive. Then convert to VDI using VBoxManage (VHD is a terrible format - do not use long term). Of course that works because almost all of my VMs are Windows, and you neglected to say what yours is.