Hi CJW,
The problem to start this topic is a different case.
"Invalid Token" has been a problem since Joomla 1.5, and until your message I thought it has been fixed with j2.5
Then i did take a look at the forums, people are still complaining about it:
forum.joomla.org/viewtopic.php?t=722111
forum.joomla.org/viewtopic.php?t=717831
This is definitely a problem of joomla. As people suggest in forum.joomla.org. Disable "cache" and try again.
If still no luck, then try to update your Joomla! the latest Joomla2.5 is now Joomla2.5.14
(DON'T EVET UPDATE TO JOOMLA 3.1.XX AS VIRTUEMART IS NOT COMPATIBLE WITH JOOMLA 3.0 YET)
Regards.