Modify existing login system to use vBulletin's user mgmt

[redtail]

The website that I am now adding the vB forum software to is login and password protected. Now, unfortunately, a user will need to log in twice to submit a vB post (once to the site and once to the vB system). I was told that it is much easier to modify the site's login functionality to use the vB login/user infrastructure than the other way around. Given this - I have a few questions:

1) Once the user logs into the site (and the user has previously registered with the vB package) - I want to log the user into the forums system as a side affect. How do I do this? Is there a call that I need to make, and I assume a cookie needs to also be set. Is there some sample code to review.

2) What other things do I need to be aware of to make this all work.

Thanks
Dave