[GameWizard]

Displaying Avatar in Header Plugin:

Hook Location: global_start
Code:

if ($vbulletin->userinfo['userid']) 
{ 
    require_once('./includes/functions_user.php'); //make sure we have the function we need 
    $header_avatar = fetch_avatar_url($vbulletin->userinfo['userid']); 
    if (!is_array($header_avatar)) 
    { 
        $header_avatar[] = $vbulletin->options['bburl'] . "/images/misc/unknown.gif"; 
    } 
}

Then place $header_avatar where you'd like the users avatar to appear. Mind you the /images/misc/unknown.gif is the location of the image if the user does not have an avatar.
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Showing user stats in the header Plugin:

Hook Location: global_start
Code:

// forum stats start  
$numbersmembers = $db->query_first("SELECT COUNT(*) AS users,MAX(userid) AS max FROM " . TABLE_PREFIX . "user");  
$numbermembers = number_format($numbersmembers['users']);  
$counter = $db->query_first("SELECT COUNT(postid) AS posts, COUNT(threadid) AS threads FROM " . TABLE_PREFIX . "post"); 
$totalposts=number_format($counter['posts']);  
$countthreads = $db->query_first("SELECT COUNT(*) AS threads FROM " . TABLE_PREFIX . "thread"); 
$totalthreads=number_format($countthreads['threads']);  
// forum stats end  

// total online start  
$datecut = TIMENOW - $vbulletin->options['cookietimeout'];  
$headerguests=$db->query_first("SELECT COUNT(*) AS count FROM " . TABLE_PREFIX . "session WHERE userid=0 AND lastactivity>$datecut");  
$headerusers=$db->query_first("SELECT COUNT(DISTINCT(userid)) AS count FROM " . TABLE_PREFIX . "session WHERE " . TABLE_PREFIX . "session.userid>0 AND " . TABLE_PREFIX . "session.lastactivity>$datecut");  
$headerguests=$headerguests[count];  
$headerusers=$headerusers[count];  
$totalonline=$headerguests+$headerusers;  
// total online end 

// get newest member name and userid start  
$getnewestmember=$db->query_first("SELECT userid, username FROM " . TABLE_PREFIX . "user WHERE userid=$numbersmembers[max]");  
$newusername = $getnewestmember['username'];  
$newuserid = $getnewestmember['userid'];  
// get newest member name and userid end

Use the following variables:
$totalonline = Total Users Online
$numbermembers = Total Members Registered

Unfortunately I don't have any other variables, so you'll need to find code for the other ones you'd like to display.