I have looked into your installation and it looks like:
- At some point under XF 1.5, an uninstall was attempted. It removed most of the VaultWiki data, but failed to remove some things like existing settings.
- When re-installing VaultWiki under XF 2.x, the existing settings were detected. Since they used the XF 1.5 format, they were not compatible. As both XenForo and VaultWiki thought this was a fresh installation, it could not handle this situation.
I have addressed this by disabling some validation when the installer generated the default values for certain settings. After this and some tweaks to ignore existing setting values while installing, I was able to get the installer to complete on your site. These changes will also be included in the installer in the next release.
Please keep an eye on the behavior of VaultWiki, not only because it is Alpha, but because more cleanup may still be required. We should try to keep this in mind when diagnosing any future problems that seem very strange.