I'm tried submitting
http://mydomain.com/sitemap_index.xml.gz and now I get this error:
"We couldn't find the Sitemap at the location you provided. Please make sure the Sitemap URL is correct and resubmit your Sitemap.
Example: http://www.google.com/sitemap.xml"
And yes I do have rewrite rules in my .htaccess (which I uploaded to the root since vbulletin runs in the root folder).