Is that variable even defined there? Located where the template is rendered in the files (do a global search in the vbulletin directory for it) and then look at the code above where the template is rendered. Do you see that variable defined? If not, you will need to write a plugin to define it first before you can use it.
You could *try* adding this line to your config.php file after the <?php and see if it works, but I am guessing it won't but it's worth a try.
HTML Code:
// Force all userinfo queries to include avatar information.
define('AVATAR_ON_NAVBAR', 1);