In Android 4.4 KitKat, Google made changes to improve security that disabled non-system apps from using the SD card. In other words, unless the app came built into your device, it can't make full use of the SD card. This unfortunately means that Vaulty can no longer be stored on the SD card of your device if you're running KitKat. We'll change this soon to allow users that have root access and have corrected SD card permissions to be able to select Vaulty's mount again. This was a feature we worked really hard to get right and we're sorry that we can't provide it anymore. We tried every way we could to get around it. If this functionality becomes available in the future, we'll add it again. We appreciate your understanding.
Update: Full SD card functionality has been restored in Vaulty on Android 5.0 or higher.