from an Oracle database sure it is possible to create a handshake script that will grab the userid/passwords and cache them so PHP can use that cached data for authentication...

the problem is that the data will have to be updated when someone register or change password on the other system, if your site is very large it can become server intense... remember that vBulletin will authenticate each user for each session... if the cached userid database is enormous you will need a lot of ram on your engine.

for newer systems, there is sometimes translations/bridges between technologies, but if your own system is old, maybe it would be time to upgrade to a more compatible version - when possible - or simply port your old system to a PHP-made system ... sometimes it cost less than keep the old system.