Hi,
Account activation, sessions, email sending, user registration (working or not working). Those are all related to Joomla itself. Only Joomla developers can resolve those problems.
We are not the developer of Joomla or Virtuemart. We are developing "template" for them. Templates are effective only on Front-page and change how virtuemart+joomla "look", not how virtuemart+joomla "work".
Be sure to update your Joomla to latest version, maybe this problem will be solved with an update. (Joomla2.5.20 is the latest Joomla release as of today).