That is controlled by, ACP --> Settings --> Options --> Forum Display Options (forumdisplay) the setting, Maximum Displayed Threads Before Page Split
Now it you do not want to change that setting you can edit the online.php file, find this bit of code.
PHP Code:
$perpage = sanitize_perpage($perpage, 200, $vbulletin->options['maxthreads']);
And change it to this.
PHP Code:
$perpage = sanitize_perpage($perpage, 200, 100);
You will have to re apply the edit each time you upgrade your site.