This is the latest trick I learned: How to mount a VDI image so that my system can use it without firing VirtualBox.
Important: VIRTUALBOX MUST NOT BE ACTIVE
Basically, you use Qemu for the trick. Try to get a Qemu version beyond 1.
Once you have Qemu, do this:
1. Check once more that VirtualBox is not running.
2. Open a terminal and go to the folder where you have your .vdi file.
3. su [your superuser password]
4. modprobe nbd max_part=16
5. qemu-nbd -c /dev/nbd0 [name of your vdi image].vdi
6. mkdir /mnt/[name of your vdi image]
7. mount /dev/nbd0p1 /mnt/[name of your vdi image]
(The blue parts correspond to the actual name of your .vdi file)
Right now, Dolphin should see the files in your virtual drive. You can do whatever you want with them, but when you're done, make sure you unmount the virtual system before using VirtualBox again:
1. Close Dolphin
2. umount /mnt/[name of your vdi image]
3. qemu-nbd -d /dev/nbd0
Done! Now everything should be as it was. Thanks to timkb4cq on the MEPIS forums for the explanation and to Jeff Waugh from Be the Signal for the original walkthrough.