I just wanted to explain how to integrate with vbulletin the easy way ..
this short how to show that it can be done and how its done ..
first install the questions to answers enabling the QA_EXTERNAL_USERS in qa-config.php
now after installing the script ..
we need to create integrate.php file ..
require_once(CWD . '/includes/init.php');
only edit path/to/vbulletin to your forum path .. for example /home/questions/public_html/vb/
and put it inside your vbulletin directory
replace qa-external-users.php with this file
download from here
edit line 51
and replace /home/admin/forums/integrate.php with the real path to integrate.php file inside vbulletin directory
and upload the file to the qa-external directory
and offcourse you need to make sure that the cookie in the vbulletin goes to the questions to answers ..
because the modification here based on the vbulletin core ..
hope this helps ;)