If one have thousands vBulletin threads to moderate, the script can fail to load them all or can fail to finish moderating action while did in bulk via "Threads to Moderate" AdminCP vBulletin section.

One can moderate threads or posts also from User control panel instead of AdminCP.

Go to yourforum.com/usercp.php

User CP -> Moderation -> Moderated Items -> Threads

And there add "&pp=200" to show example 200 topics per page. You can select All and do the Moderating action.

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If one get errors like Fatal error: "Allowed memory size of X bytes exhausted" one need to raised "memory_limit" PHP option (in php.ini file) .. google it

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SECOND WAY:
In AdminCP go to Threads & Posts / Prune / and select to delete threads awaiting for moderation. This will delete them all permanently..!