You DO need a license. OR it won't run. If it's a pre-4.0 version, you can likely hack it and VB won't care. Here's how I know this.
Dig around the vBulletin forums. When 4.0 came out, they have basically said you can use pre 4.0 versions forever, without recurrent licensing fees, but also there will be no further support for it. The documentation will still be maintained. It took quite a bit of digging to locate and verify this, but if it's a pre-4.0 version, it's yours to use as you like, forever. Security issues? LOL . . . minimal, if you install it correctly. Previous thread [webmasterworld.com] describing VB's move, which I think was not a very good one.
Thanks rocknbil for your comment. I use latest vbulletin 4.0 gold brand without powered by vbulletin sentance. I have cheaked each and every bit of function, and each works greatl without license. But suppose, in future if vb staff notice my forum site without license then they can take action against me? (remember there is no powered by vb in my site)