While vBulletin is nice, I have my doubts that it is any more secure than phpBB. phpBB is a well-written application, and I think it will become more and more secure over time, not less - the key thing is that they have a development team that is vigilant about closing these holes quickly. A lot of these site defacements come through vulnerabilities in other software like PHP itself or various add-on packages, etc. The bottom line is that the more publicly available software and scripts you have running on a site, the more vulnerable it is. APC has quite a lot of special features and custom modules, so it's something to watch out for.
No complex web software (vBulletin included) is 100% secure. It is very likely that there are more undiscovered security bugs in both phpBB and vBulletin. You don't want to let your guard down or assume you're protected just because you're running commercial software now.
FWIW. I'm a grad student specializing in computer security and I've read in detail about how the recent phpBB problems worked.
No complex web software (vBulletin included) is 100% secure. It is very likely that there are more undiscovered security bugs in both phpBB and vBulletin. You don't want to let your guard down or assume you're protected just because you're running commercial software now.
FWIW. I'm a grad student specializing in computer security and I've read in detail about how the recent phpBB problems worked.