Do you want to delete all your data for privacy reasons or do you want to reset your Vox to factory because you rooted it?
If 1) just go into Settings - Storage - Unmount SD - Format SD (or do it on a PC, same thing) and Settings - Privacy - Factory Reset.
if 2) it's a very tight spot - took me 20 minutes to get it, but I returned a totally virgin Vox (my backlight bleeding had gotten worse). Power off, then power on until the green light blinks and immediately hit power again for 2 seconds or so. The green light will continue to blink. You're probably in some kind of recovery now, but the display won't work. I believe from there it is 2 times power and Volume Down and the thing should reset completely (apply the system image that is saved on the recovery partition). Anyhow, the Kobo recovery is more intelligent than the normal Android recovery - I guess just making trouble (pressing stuff in that screenless recovery) should make it kick in as well.