Integrating external script with vBulletin users

[Arven]

Hi!

I tried to use search to solve my problem (as well as looking into tutorials about hooks, plug-ins and the like - but all in all it's too much and i need some help getting started in the right place).

So here's the thing:

A few years ago I programmed a deck building data base for the Warlord: Saga of the Storm CCG. Fast forward a few years and now I am actually in charge of the product, am running the official forum and want to integrate and upgrade version of it into the official forum, so registered users of a certain group can automatically access the advanced functions of the Deck Builder when they go to the forum (and especially don't need a bunch of different log-ins).

Adding a link to the Deck Builder site is easy, but how do I go about using vBulletins user management in a "plug-in" (it's more like a "tag-on" ;-))?

Can anybody point me to the right start? I am willing to read and learn ;-).

Thank you!