[CoffeeLovesYou]

Make sure you have your Plugin/Products enabled in vB Options.
If you go, in the ACP, go to Products/Plugins and Add New Plugin. Make the name of it whatever you want (i.e. Minimum Chars for Forum ID xxx) and for the hook, set it to fetch_foruminfo in the dropdown box. Leave execution order how it is.
Paste that code Lynne provided into the plugin code.
For example, if you want ForumID 3 to have 5 min chars, the code will be:

PHP Code:

if ($forumid == '3')
{
    $vbulletin->options['postminchars'] = '5';
} 


If you wanna do more than one forum, just keep adding them on.

i.e.

PHP Code:

if ($forumid == '3')
{
    $vbulletin->options['postminchars'] = '5';
}  
if ($forumid == '32')
{
    $vbulletin->options['postminchars'] = '1';
}