3.6.4 has multiple security vulnerabilities in it.
http://www.cvedetails.com/vulnerabil...tin-3.6.4.html details all of the vulnerabilities in it.
http://www.1337day.com/exploit/1545 - PoC demonstrating SQLi vulnerability.
It's important to stay up to date with any sort of publicly available system and definitely something as large as vBulletin. Your version is from 2006 -- that's almost 10 years old.