[BirdOPrey5]

Go to plugin manager -> add new plugin
hook: forumdisplay_start
title: Show all Threads in Forum
Execution Order: 5

Code:

if ($foruminfo['forumid'] == 57)
   $perpage = $vbulletin->options['maxthreads'] = 200;

Where 57 is the forum id number of the forum you want to show all and 200 is the absolute maximum, make sure it is well above your actual number of threads but if it's too high and you really have a lot of threads it may cause performance issues... so don't do this if you have thousands of threads or something.

If you want to do this for multiple forums use:

if ( in_array($foruminfo['forumid'], array(57, 58, 59)))
   $perpage = $vbulletin->options['maxthreads'] = 200;

Where 57, 58, 59 are multiple forum ids...