multiple accounts per user tie to one table of users

[esiaz]

Hi, i am using vbulletin 3.6.6

Currently i have a table call app_table in my vbulletin database which contains a set of users. Each of this user can have up to 3 accounts (1 account of 1 user role). Our rule here is that one person can only have 1 user role(admin,mod or user).

after login.php?do=login, it will proceed to a page call selectrole.php. At this page it will display 3 user accounts with buttons for user to choose which account he wants to log in into.
I have successfully passed in the $vbulletin->userinfo['userid'] to index page after the person clicks on the desired user account. But somehow session has logged the account to the last account in selectrole.php no matter which account the person chosed.

I have traced the problem up to vB_Session function in /includes/class_core.php but i do not understand how to log in the chosen account.

Can someone help me pls~ Previously nobody answered my other enquires. Hopefully somebody can help me this time.

Thanks!