Use the User datamanger....
Heres a simple ex.
PHP Code:
$newuser =& datamanager_init('User', $vbulletin, ERRTYPE_ARRAY);
$newuser->adminoverride = true;
$newuser->set('username', $username);
$newuser->set('password', $pass);
$newuser->set('email', $email);
$newuser->set('usertitle', $usertitle);
$newuser->set('usergroupid', $usergroupid);
if(!$newuser->errors){
$newuser->save();
unset($newuser);
}
If $newuser-errors is set it will be an array of all errors that occured