Login + session table question

[dRiFT]

Hi, I'm working on a modified login for a vB forum, this is to get it using the same login details that already exist for the web site (email addy + password as opposed to username and password).

All that side of things is working fine, the site members automatically get a forum account created when they join and the first time they login in they're asked to specify a forum display name(custom template) which it then validates against the usual vB name rules. However if you choose not to bother specifying a name when asked and try and visit the forum front page it's logged you in and you're given the name 'unregistered' as the field's blank in the db. This is where the problem lies as I don't want anyone being able to access any member stuff without giving that display name.

So onto the solution...

What I'm trying to track down is the point at which the users session info has their userid added to it, so that if the display name is blank I can stop this being done and therefore they appear to still be logged out, until they return and actually bother to enter a forum name. It's proving to be a bit tricky when you can't get the forum to output any debug values simply. Has anyone tackled this kind of problem before? As always I'm trying to achieve this with the minimum number of mods to the system.