[nathan-mahdavi]

This is a working script (tested on VB 5.0.1) that will create a random user and then log them in using their ID. If you already know a users ID then you can just log them in using the log_user_in function.

PHP Code:

<?php

chdir("forum/core/"); // change to your core folder location
require_once("global.php");

function create_random_user(){
    $dataman =& datamanager_init('User', $vbulletin);  
    $dataman->set('username', "user".rand());
    $dataman->set('email', rand()."@domain.com");
    $dataman->set('password', "leighf");
    return $dataman->save();
}

function log_user_in($userid){
    $user = fetch_userinfo($userid);
    $auth = vB_User::verifyAuthentication($user['username'], "leighf", $user['password'], $user['password']);
    $res = vB_User::processNewLogin($auth);
    vbsetcookie('userid', $res['userid']); 
    vbsetcookie('password', $res['password']);
    vbsetcookie('sessionhash', $res['sessionhash']);
}

// create random user
$userid = create_random_user();

// log them in
log_user_in($userid);

It's a script in its simplest of states. The usernames password is hardcoded but assuming you know what's going on in the above snippet, you'll be fine adding this yourself.