[BirdOPrey5]

Make a new plugin on the hook forumdisplay_start with this code:

if ($foruminfo['forumid'] == 2)
   $perpage = $vbulletin->options['maxthreads'] = 200;

Where 2 is the forumid and 200 is the number of threads you want per page.

You can add additional lines for different forums-

if ($foruminfo['forumid'] == 2)
   $perpage = $vbulletin->options['maxthreads'] = 200;

if ($foruminfo['forumid'] == 3)
   $perpage = $vbulletin->options['maxthreads'] = 100;

if ($foruminfo['forumid'] == 4)
   $perpage = $vbulletin->options['maxthreads'] = 250;