External login problem using vBulletin database
 

I am trying to integrate user info and data to my main site (custom PHP code) and I managed to do it using data from "vb_session" table, by matching the idhash record, but there are circumstances where a user has no data in that table and he is not recognized by my code, yet if the user first visits the forum (the data is created in the vb_session table) and then he returns back to the main site he is recognized.
I presume that there is some kind of mechanism (via cookies?) that vbulletin creates user entries in vb_session table.

So my question is how to trigger the auto-login process if there are no session data in vb_session table - how is vBulleting doing that.

Any help or guidelines would be appreciated, I am in a dead end.