Pirated or "nulled" vBulletin websites always have a backhole. The users that null the forum software are usually just a bunch of script kiddies that go into the source, look for callhome scripts and remove them and distribute them. But, in the process, they remove some legit code that protects their sites.
Fail, right?
Where are you getting your nulled vBulletin from? I assure you there are plenty of copies going around without backholes. Just because it's warez doesn't mean it's automatically not secure. I've personally seen copies without back holes going around..I know because I've downloaded them and compared them line for line with a legit version. All most of them remove is the .js call home in the admincp and the step during installation where you have to input the customer number. I've even seen copies floating around that supply you with a customer number to use during installation..I assume whoever released it just modified a certain constant in a .php file.
My point is vBulletin is easy to nullify and anyone that can read the source code can do it in a couple of minutes. Yes there are warez copies with backholes floating around the internet but they are the minority. Most warez groups pride themselves on releasing good software, not crap.
Getting back on topic; I stopped caring about my modifications begin on warez sites ages ago. If it bothers you I advise you to send a DMCA notice and hire a lawyer because you're going to have to do it yourself.. it is your responsibility to protect your own IP if you choose to do so.
You'll never stop the warez scene so your best bet is learning how to work with it. Keep in mind that someone using a pirated copy doesn't automatically mean you lost a customer (assuming you code for money). Most people that pirate wouldn't have bought your software anyway.