Strange problem with plugin

I’m trying to get a vBulletin username to display in the wordpress header.

vBulletin is running on the site root and WP in a subdirectory. I need to include vB’s global.php to get the username.

The top of the plugin script has the following lines:

$vwd = '/usr/******/docs';
$cwd = getcwd();
chdir($vwd);
if (!file_exists($vwd . '/global.php'))  { echo "cannot include $vwd/global.php.  You should check your path!"; exit; }
require_once($vwd . '/global.php');
chdir($cwd);

However, when trying to activate the plugin, I get a fatal error and the plugin stays de-activated. If I comment out the “require_once” line, it activates fine. If I then uncomment it and re-upload the plugin script, everything works fine EXCEPT if I try to edit settings of other plugins I have installed (then I get a message “Cannot load 0”).

Any idea what’s causing this strange behaviour? Or any idea how I can better integrate the vB username into my WP templates?

Many thanks!