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