Well the basics for integrating with vB are very simple, just include the following line in your script:
PHP Code:
require_once('./global.php');
This will give you access to all the userinformation, including the groups a user belongs to. Just look into the $bbuserinfo array that will be set after you include global.php.
When your script is located outside the vB homedirectory, you will have to chdir to the forumhome dir before calling global.php, and depending on your script chdir back afterwards.
This should get you started. If you have any more specific questions, feel free to ask.