[kh99]

Maybe this is stating the obvious, but it looks like the mod you linked to does not change what external.php returns, it just gives you an alternative place to link the RSS icons to. So it seems like you should probably disable your external feeds then find all the places that include a link to external.php and change it to the appropriate feedburner link.

Doing a search of the vb files, it looks like there's one place in ajax.php that uses external.php. There's also a phrase called rssposter_edit_url_text but I can't find anywhere that it's actually used. The rest of the places can be found by searching the templates for external.php.

If what you want is to have your external.php return the feedburner feed, you may be able to have external.php return a redirect to feedburner.com like is done with external.php in the ajax.php script.