I found
this thread about it over at vBulletin.com (also read the second page, the last post is the most interesting one). It seems that the option was removed in a specific version of vBulletin 3.5.x and was re-introduced in vBulletin 3.6.1
So I would suggest you to upgrade to the newest version, vBulletin 3.6.2. It's very easy/safe and there are a lot of new features