I'm not familiar with Vbulletin but very familiar with phpbb. I know out of the box phpbb is not very search engine friendly and I believe Vbulletin is the same way.
phpbb has modifications available to address these problems. I know the best one for phpbb3 is not listed under the mods database on phpbb.com It's an entire suite of mods, url rewriting, no duplication, unique meta tags etc.
vBulletin has a more powerful admin panel, and seems more spam-resistant.
For a while, phpBB was plagued by security exploits and frequent patch requirements, but that seems to have settled down.
Invision Power Board (IPB) could be an option. I'm swithering between IPB and VBulletin (both cost around the same up front).
VBulletin, IMO, was always the best, but it seems IPB has come on leaps and bounds.
Rogered, check out phpbb3 admin panel. ;) Spam plagues just about any forum software. Haven't seen any reports of spam yet that I'm aware of for phpbb3 but that is sure to change. The captcha system is much harder than the one for phpbb2 which BTW is adjustable. Still not adequate IMO because it still relies on an image.
No patches for phpbb2 for about year now, think that speaks for itself.
vbulletin all the way, I have tested majority of forum software scripts and till day only satisifed with vbulletin.
When it comes to features, technical support and available modifications.
Of course vbulletin..
If you don't have money (for vbulletin license) use SMF,
if you know php(i mean if you can develope something with php) use phpbb.
I think, you have to think like this:
if i buy license for vbulletin, in how many days will i take my money back. (with adsense or something else)
Vbulletin is the best forum system i think..
Buy license and vbseo. Enjoy it =)
If you don't want to pay for SEO to vbseo, you can use zoints seo system.
Regards from Turkey,
(Sorry for my bad english)
vBulletin by miles
SMF - It's all you need :)
SMF is a GREAT tool. But it takes a HUGE amount of space
vBulletin is the best option. It has great administration features, and is easily extensible. The plugin and product system is designed in a way to minimize and eliminate alterations to the core system, meaning that when a new release or patch comes out you don't have to go reapply all of your hacks.
The issue with the plug-ins as I understand it is that they cause a lot of resource usage.
As far as phpbb2 goes you never had to reapply your hacks, at least for the last 3 or 4 years. With each update they have also released a mod file that is basically a hack itself to apply the changes to your current files. If you have easy mod installed these usually go off without a hitch so for most updates you're talking about minutes, there are exceptions of course. Really no big deal because you just don't complete the installation and manually edit the changes where the conflict occurred.
As far as phpbb3 goes it has similar update system built in, I haven't used it yet but from my understanding it automatically applies updates to files that have not been modified. Files that have been modified you're given a notice for and you can either back out of the update to apply manually or have the updater do it.Apparently it shows you exactly what lines its going to add or edit. I believe there is even an option for manually doing it through updater itself. phpbb3 does come with a file editor so I don't see why they wouldn't take advantage of it.
Take a look to bbPress. Its made by the same guys that did Wordpress. Its search engine friendly, template based, easy to modify if you looked into Wordpress code and handles well spam with akismet.
Hope it helps.