depending on your clients needs, you have two solutions:

1- code a bridge that will use vB or the other systems login/authentication details to be logged across the system. an example is Photopost.com... all the scripts there use vBulletin login to swithc the user authentication.

2- completely switch to vBulletin if you can... vBulletin is by far, one of the most reliable structure to integrate websites and older content by importing or merging old content...