You can download the patch for your version here: http://members.vbulletin.com/patches.php
As part of our maintenance of Cloud Sites, we will apply the patch to them. If necessary, we will contact Cloud Customers with further information.
The current vBulletin 5.1.3 Alpha will include this fix on its next release. vBulletin 5.1.3 Alpha should not considered suitable for production or live servers.