I haven't tried it, but maybe you can override the user's autosubscribe choice depending on the forumid. For example, maybe try using a plugin on global_setup_complete and something like:
PHP Code:
if (THIS_SCRIPT == 'newthread' or THIS_SCRIPT == 'newreply' AND $foruminfo['forumid'] == X)
{
$vbulletin->userinfo['autosubscribe'] = 2;
}
Again I haven't tested that, it might not work as is.