I presume you mean an unlicensed version? What actions did Jelsoft take?
Jelsoft is quite aggressive in protecting their copyrights, and as others have mentioned, they will contact your host and ask to have you site removed under the DMCA.
I found this out the hard way, as I was unintentionally running a forum that wasn't legal. They had sent the site's own renewal notices on her leased license, but her spam filter ate them before she saw them. After several tries and a month or so of no response, they took further actions.
I mention this as a warning to any of you running the leased version (instead of owned). It's easy to loose track of when your lease is up for renewal.
On the other hand, I've found the technical support and community at Jelsoft and VB to be second to none. I've used it for several years now, and consistently gotten prompt and accurate responses to problems and questions. They listen to their customers and often add features based on members requests. This is a robust and solid program, well suited for a production environment. I currently manage 5 different forums, and they're all done with Vbulletin. Two of them routinely have 100 members or more online at once, with no problems. All in all, I highly reccomend Vbulletin.