[ErnestA]

You must enter the full path (not relative). Let's assume you have your forums installed as:
http://www.yoursite.com/forums/

I.E. your entire vBulletin install is located under the forums subdirectory of your web root.

Go into the forums subdirectory and create a directory called sitemaps

Be sure to chmod 777 the sitemaps folder... If you can't 777, use 755.

Your sitemap files would now be visible under http://www.yoursite.com/forums/sitemaps

Now, I don't know who your hosting provider is, but I use HostGator and the full directory to my sitemap folder is:

/home/myaccount/public_html/forums/sitemaps

This is the path you put into the sitemap options in vBulletin.

If you're going to upload your sitemap to Google's webmaster tools vBulletin uses a script called xmlsitemap.php to serve your sitemap files

So, the path you would give to Google would be http://www.yoursite.com/forums/xmlsitemap.php

Regards,

ErnestA.