[kh99]

If you're doing your add in a plugin or a custom page using vbulletin code, then you should use the forum datamanager (in includes/class_dm_forum.php). You can see how to use it in by looking in admincp/forum.php, the section that starts with:

// ###################### Start update #######################
if ($_POST['do'] == 'update')

I don't know the answer to your question offhand, but looking at that code a little bit and when the datamanager saves a forum it calls build_forum_permissions() in includes/adminfunctions.php, and I believe that also rebuilds the forumcache, which is probably your problem. If you can't include the vbulletin code so that you can use those fucntions, then you might want to look in there and see what it's doing.