[Opserty]

Try this:
AdminCP > Plugins & Products > Add New Plugin > Set:

Product: vBulletin
Hook Location: showthread_getinfo
Title: "..." - Whatever you want
PHP Code:

PHP Code:

if(($threadinfo['forumid'] == X) AND ($_GET['postorder'] != 'DESC'))
{
    $postorder = 'ASC';
} 


Is active? Yes

Replace X with the forumid number of the subforum (You can find it in the Forum Manger).

Haven't tested it but it should work. Also then if users want to override it they can add "&postorder=DESC" to the URL.