Planning what you want and Knowing what you need are the key factors in adding tags onto your forums.
Adding CSS to your forums will require you to add multiple different tags, rather than just one such as the [css-div] tag.
Well, you could add just that one tag, but it would be a rather bland addition without some other CSS enabling tags.
vBulletin's manual explains every inch of managing a vB forum; including how to add custom BB code tags.
It isn't hard at all, you should give it a look sometime.
If you need an example of tags added in a vB forum (Pokecommunity being a big example), check at the very bottom of a thread.
You'll see a box labled "posting rules", and in it you'll notice that BB code is set to On, with BB code linking to another page.
If you click it, it takes you to a list of available tags to use on the PC forums as well as a small example explaining how to use them. It should point you in the right direction of knowing what tags you would want to add onto your forums nonetheless. :)
Now, for the addition of CSS tags onto your forums, you will have to know a hefty amount of coding.
If you aren't coding savvy, you should search around the vBulletin community for further help.