Sharing some user account details on concurrent installs

[Boards.ie]

Hi,

I'm migrating one of our systems from Photopost classifieds to vBulletin for a variety of reasons. We already run a large vBulletin-based site (not heavily modified) and would like to share the profile (username/password/email etc) details of the current vBulletin site with the new one (as the Photopost site is a "subsite" of the primary vBulletin site and already does this).

I'd like to know if there is a relatively simple way of sharing these details (as opposed to copying the details on a cron or something, not feasible with the size of the database). I would like to avoid having to modify the vBulletin codebase manually (it would suit me perfectly if there was a mod for this, but I suspect it is rather too niche).

Perhaps this can be done by symlinking MySQL data tables; could you advise on the best course of action? I would prefer if it was possible to specify by database/table in a config file for the sake of portability.

Many thanks for any assistance.