How to change the maximum number of vBulletin threads per page?
To change the maximum number of threads that will be visualized on a page, go to your vBulletin admin control panel > Settings > Options > Forum Display Options (forumdisplay) > Maximum Displayed Threads Before Page Split.
If you have more than the maximum number of threads a navigation link to the next page will be shown under the last thread. Using the links in the corresponding navigation area allow you to scroll between the pages with threads.