If you already have a database of users independent from the forum, it gets kind of tricky. For shits and giggle's, I'll explain how to do it a few different ways.
Solution A.) Build on top of the forum.
It's as simple as using the forum's database from the start, and building your custom site around the database the forum already has. This involves a heck of a lot of reverse engineering- including thought and much accommodation for the way the forum's database is organized. The downside to this approach is that commingling your own code with already established code from the forum may lead to name collisions, and several confusions regarding where your custom code/database ends, and the forum's native code/database begins. If you want a very tight connection to the point where the users (and any internal programmers) don't know where your site ends, and the forum begins, this solution is for you.
Solution B.) Build along side of the forum, linking the two on events.
This solution is much more difficult. You need to heavily reverse engineer the forum, and write linking scripts that are designed to execute when an event occurs on the forum, or on your custom web app. For example, if someone logs in on your forum, you need to find the login code (hopefully only one piece of code!), and mark the same user on your custom database as logged in. You need to do the same for a log out event. Now let's say you have two registrations (forum and your own web app). You need to make sure the credentials required on the forum and your custom web app are exactly the same, and link the forum and your web app whenever someone registers an account on either. The more you want to combine the two systems, the harder this solution becomes. The upside to this solution is if you already have a well established system, and recently added a forum. If your current system is so well established that it is easier to have a bunch of linking routines than it is to start fresh and build from scratch on top of the forum's existing architecture, this solution may be for you.
Solution C.) Search for someone who has already done what you are trying to do.
This solution assumes you are just starting your project, and can afford to start over. Orr, you are lucky enough to have started with a popular system, and someone else was kind enough to write linking code for vBulletin => your CMS. For example, someone has already combined WordPress with vBulletin, and why bother making this more complicated than it needs to be, when you can simply use WordPress and vBulletin? There are tons of freelancers who are experts with WordPress, and several freelancers who are experts with vBulletin- so if you're not the programming type, this solution is probably for you. Don't like WordPress? It's just an example! If you are using another CMS already, see if someone has integrated it with vB already. If not, but you can afford to change now because your project is still in an early stage, consider switching. Ask yourself if this solution is for you- because it's the simplest route for a non-programmer- and best of all, prevents you from being tied down to the one who gets you into the mess you're going to be in when (s)he's done coding Solution A or B. Even for a programmer who simply doesn't want to bother with all the aforementioned reverse engineering madness.
