You can check it out again from within the Aldiko app. I have a book that I downloaded to my computer a couple of days ago that I was able to add to the Vox and read with Aldiko...I don't use Overdrive at all. Just the Overdrive section of the Toronto Library to which you must first sign in (through the catalogue in Aldiko) and then find the book & download. The catalogue url to use to enter the Overdrive section of the TO library is below (on the off chance that you borrowed your book from there.
http://toronto.lib.overdrive.com/