am getting the same error! i heard it's a bug with non-english forums (using another language pack instead of english) ..
what i did and i know if it's the right fix, but it worked with google sitemap..
- i download (vbulletin_sitemap_forum_2.xml.gz) file
- edit the old dates and change to new date & time (2010 !! )
- upload it again to the (sitemap) folder
- resubmit it .
-

appeared next to it!