Fatal error: Class 'VirtuemartControllerXYZ not found in /components/com_virtuem

[Milbo]

This error comes from the SEF and shows that the parsing of the SEF link does not work. The solution is here to

 assign vm to a joomla menu item.

What means this? Go in your joomla administration and to the menue marked with the star (your home, your default menue). Create a new item there and let point it to the virtuemart default layout. The link should show "index.php?option=com_virtuemart&view=virtuemart".

Maybe some more information about this here
http://forum.virtuemart.net/index.php?topic=91602.0