When you connect the Vox to your computer using USB, the Vox itself dismounts the SD cards, so they can be mounted by your computer. Aldiko is expecting the SD card to be there, tries to access, can't, and crashes. To get Aldiko back, you must reboot the Vox.
Those are my theories, and I am sticking to them! (for now, until better ones come along.)
Mantano reader seems to do this less often (or maybe I am just not connecting to the computer very often any more). Try it. You can get it at:
http://slideme.org/application/mantano-reader-free
Keith