For Kindle I use Calibre, load them into here and then sideload them onto the Vox, not allowed to discuss how on here, but if you search for Aprentice Alf in gives details
If you want an easier way, go to the link mentioned above on using your Vox browser, which is
http://goodereader.com/apps/
Near the bottom you will see Kindle for Android
Click on this and you will see a button to download, click this
If you now click at the top of the screen and drag down (this is the notification area), alternatively press the middle button at the bottom and press Notifications
This should show the download for Kindle, when it shows complete, click on it and it will ask if you want to install it
from then you can press the All Apps button on your home page and run Kindle