This was a linux laptop. I immediately powered off and booted with a live CD. To be honest I'm not sure exactly what I did next. I might have just done "less -f /dev/sda5". I think what I actually did was make an image of the partition using dd to an external drive so I would be able to recover the file at leisure. Trying now, it seems that vim won't open a /dev block device directly, but an image is just a regular file.