This doesn't look like the cause of the error is due to the plugin.
The class JVersion is defined in the Joomla library file
libraries/joomla/version.php
It is a utility class that Joomla uses so that it know which version it is. It is automatically included in the Joomla framework.
Possibly there is a problem with this file being missing from your Joomla libarary - occasionally I have come across cases where people remove part of the joomla library thinking that they don't really need it. Yes it is needed, so don't remove it.
It is possible also that the vBulletin login plugin does not include the Joomla framework correctly, therefore JVersion would not be defined. The master user plugin should work with all plugins that support the Joomla authentication plugin framework, but there is no reason why it would work with some arbitrary plugin that just happens to log you into Joomla. It sounds as if the most likely explanation is a defect of this sort with the vBulletin plugin.
It may be possible to get around this. JVersion is just used to detect which version of Joomla is being used. Obviously you know which version you are using, so you can remove that part of the programming logic, and just use the code that applies to your site, eg change
$version = new JVersion();
if($version->RELEASE == '1.5')
{
jimport('joomla.event.plugin');
$this->_plugin = JPluginHelper::getPlugin('authentication','masteruser');
$this->_params = new JParameter($this->_plugin->params);
}
else
{
jimport('joomla.plugin.plugin');
$this->_params =& $this->params;
}
to
jimport('joomla.event.plugin');
$this->_plugin = JPluginHelper::getPlugin('authentication','masteruser');
$this->_params = new JParameter($this->_plugin->params);
if you are using Joomla 1.5
jimport('joomla.plugin.plugin');
$this->_params =& $this->params;
if you are using 1.6
