anywhere. As follows:
chainloader (hd0,1)/vdiskchain vdisk=/MyDir/Ubuntu.vdi.vtoy
chainloader (hd1,2)/abc/vdiskchain vdisk=/MyDir/Ubuntu.vdi.vtoy
vdisk parameter is the full path of you vtoy file (only path from root. Don't include something like (hd0,1) (hd1,1) ..... )
It will use grub2's search command to find the partition for vdisk, so make you vdisk fullpath unique in your system.
Thank you and using also your description
- vdiskchain is working OK to boot Ubuntu.vhd.vtoy from SSD USB in UEFI Secure mode and in MBR mode.
In my case files vdiskchain and ipxe.krn are located in grub rootfolder of FAT32 USB Boot drive and Ubuntu.vhd.vtoy is located on NTFS drive of portable SSD USB.
\grub\grub.cfg menu entries were used
if [ "${grub_platform}" == "efi" ]; then
menuentry "Ubuntu.vhd.vtoy - /grub/vdiskchain" {
chainloader /grub/vdiskchain vdisk=/Ubuntu.vhd.vtoy
}
else
menuentry "Ubuntu.vhd.vtoy - /grub/vdiskchain" {
linux16 /grub/ipxe.krn vdisk=/Ubuntu.vhd.vtoy
initrd16 /grub/vdiskchain
}
fi
-
Also in MBR mode direct booting with Grub4dos is working OK
\menu.lst entry
title Ubuntu.vhd.vtoy - /grub/vdiskchain
kernel /grub/ipxe.krn vdisk=/Ubuntu.vhd.vtoy
initrd /grub/vdiskchain
Very good that Ubuntu 64-bit Linux in VHD file is bootable from USB in UEFI Secure mode using Grub2 and in MBR mode using Grub2 or Grub4dos 
It allows Multi-Boot of Windows and Linux VHD in an easy way without the need to change partitioning of your computer.
I have modified my internal hidden EFI drive to boot in UEFI Secure mode with Grub2 and use vdiskchain to boot Ubuntu VHD located on internal GPT NTFS drive.
Nice thing is that the WARNING with 10 seconds delay does not occur when booting from internal SSD in UEFI Secure mode with Ubuntu VHD 
For Ubuntu this procedure is working OK, but for Kali Linux 64-bit Installer
the Kali Linux VHD prepared with VirtualBox fails to boot in UEFI Secure mode from internal SSD or USB SSD (security problem for normal module).
The UEFI version of Kali Linux VHD fails to boot in MBR mode from USB (UEFI version Not compatible with MBR booting).