I just tried to create a Category, a Forum, and a Link Forum at our XenForo site and I am unable to reproduce this problem.
Please check the Development tab in your Admin area and make sure VaultWiki's Code Event Listener for load_class_datawriter is present and enabled. Also check if you have any other add-ons installed that have a load_class_datawriter listener. If it adds to the Forum writer's _getFields method without calling parent::_getFields inside, that would also cause your problem, but it would be a bug with the mod that does that.