Questions about integration

[foxxcorpintl]

Hi,

My situation is as follows:

I paid a programmer to build a custom section of my website. The section allows for the posting of reviews on a certain type of product, similar to Amazon.com, etc. I have also built a great homepage, logos and everything else. I want to build more of a community than a site for product reviews. This requires a classifieds section and a forum. Currently, users log in with a username and password, handled with SQL tables and PHP. When the user logs in, he hits the landing page from where he can find reviews on products.

THE PROBLEM:
My problem/issue/question is the following:

Can I integrate the existing user login scripting and pages with Vbulletin? Ideally, I want the user to log in from anywhere on the site - classified section, forum or review section, and be logged into everything. I cannot have a situation where the user has to log in to the review section, then wants to post a thread on the forum and have to log in seperately, with a seperate user name and password (or register again if doing this for the first time). Also, I would like everything integrated to one landing page, so users can see their forum posts, classified ads, product reviews all from their landing page.

I am VERY open to ideas / suggestions and I do have some fair knowledge of scripting, PHP and SQL.

Thanks!