Actually it is quite the opposite.
Vbulletin has a shitload of holes, compared to phpBB which can have its security tighten a lot more clastrophobically.
Also vbulletin builds up by stealing from just about any other successful BB which has been around the block way more than they had.
Not to mention, that last I checked, it's liscence agreement denies you the legal right of customizing it to your needs, IE no modifications to the source code are allowed.

"Knowledge is ultimacy. We are knowledgeable, thus, we are supreme!" - Ori"Gravitron"Klein