I was having another problem with my system as a whole, and it was fixed by upgrading my Joomla! 1.5.9 system to Joomla! 1.5.12. However after that was done I started having trouble with virtuemart admin. In Joomla admin session, VM goes back to login when any option is chosen. That is, when you pull up the VM admin control panel any item you choose brings you back to the admin login screen. No other component has been affected as far as I can tell.
I have tried disabling all the plugins aside from the required ones for Joomla! itself and it didn't make any difference. I tried turning on the VM debug mode but that didn't show anything it just went to the login screen without any status displayed.
I am running VM 1.1.3.