Be root and do:
modprobe nbd max_part=16
qemu-nbd -c /dev/nbd0 <vdi-file>
mount /dev/nbd0p1 /mnt
Use /mnt as you may.
Finally do:
umount /mnt
qemu-nbd -d /dev/nbd0
Idea: <link http: bethesignal.org blog how-to-mount-virtualbox-vdi-image external-link-new-window>bethesignal.org/blog/2011/01/05/how-to-mount-virtualbox-vdi-image/