Nulled vBulletin is not a different version of the forum software vBulletin. Nulled vBulletin is a version of the software that has been hacked to remove the license information. The nulled means it has been pirated and everything that could identify the owner of the licence has been removed or deactivated (however, sometimes the hacker does not do a good a job as the think they have). The call-home function to identify the version of the software is also usually removed. Nulled vBulletin software is illegal. There is only one way to get vBulletin and that is to buy it from vBulletin for $195 (which is not that much, so not sure why so many go to so great a length to get an illegal version).
The advantage of using a nulled version is that it’s free. The disadvantage is that it probably has more security holes in it than a purchased version; the people who nulled it may have put in some backdoors for themselves; there will be no official support from vBulletin; you are breaking the law, which may catch up with you; the host may close your site down without warning; if you use something like Adsense, then your account with them may also be in jeopardy if vBulletin take it up with them. There are a number of clips on You Tube about how easy it is to hack a nulled vBulletin, so good luck using it.
If you know a site using nulled vBulletin, it can be reported here: http://www.vbulletin.com/piracy.php
The good news is, that if you are using a nulled version, you can buy a current licence and upload the files and your forum will keep on running.
If you want to see how dangerous using a Nulled vBulletin can be: