Thanks for the help as always everybody.
That being said, I have been informed that the project was not correctly described to me the first time around, so I need to restate the issue.
While we have enough PHP programmers in house to accomplish a bridge (and thank you guys to letting us know that is possible

), we do not know the vBulletin passwords nor user names that we would need to know in order to develop the seamless link (or SSO) from the already established user names and passwords from our website to vBulletin. That being said, does anyone know of a seamless link/SSO solution to such a problem?
This is a tall task, so I am not sure a solution exists, but i just thought I'd continue to throw that out there and see if anyone had experience in a similar situation.