The Vox actually has several internal drives (they're actually called partitions), only one of which is visible to the user. Formatting the SD card, either using hieronymos' or Reader Paradice's instructions removes everything from that drive. Doing the factory reset clears one of the non-user visible drives (I think). That should be sufficient to remove any data/programs that you added or modifications that you made to the settings. It is not sufficient to remove any hacks that you may have made -- but you don't have to worry about that if you didn't go that route.