[dstruct2k]

It is quite possible; as long as both integrated systems are looking for the same information in the same cookie on the user's computer, they will both authenticate.

This will require modifying one of the two systems to use the method the other one uses for authentication. I suggest that you look into modifying your custom system to work with the vB user database, as your copy of vB will not be able to recieve official support if it has been modifed to the extent required to read a third-party user database instead of its own.