Single login across multiple WP installs AND pre-existing site/vBulletin

I am currently working on porting a site over from an aging custom CMS to WordPress, in addition to adding a blog to the pre-existing site (to be launched before completion of the transition to WP for the site-at-large). This site currently has its own user system and a vBulletin install, whose logins are essentially integrated. I did not write the custom CMS or the existing login script, but have access and know how they work. The users of the blog and the general site may not always have the same privileges, so they should probably not share a user database. Both WP installs do authenticate from the site’s user database, using the External DB authentication plugin.

The powers that be would like to have integrated login between all areas of the website. How can I log users into two separate WordPress installs in different folders at the same time while also running our existing login script? Is it not possible unless the two installs share a user database? The current login script authenticates with vBulletin first, and then when the user goes back/is redirected to the page they were previously viewing, session variables are set based on those credentials.

I’ve managed to get logout working for one install, but it required editing a core WP file because vBulletin uses a logout hash as a $_GET variable, which was getting lost in the redirect. I am still battling how to log out of two sites at once, so any help in that area would also be appreciated.

Thanks in advance for this mess…