Just going with the two products you listed.
vB has the largest quantity of addons, as such a large number of them are free (and many also likely abandoned and not receiving updates). It also has a fair amount of good skins, but the load times leave much to be desired, especially in their recent vB 5. Honestly, I would avoid vB because of that; if you want to run a solid forum with access to most of the addons that have been released over the years, you'll need to install vB 4 which is no longer receiving major updates (still receiving security fixes, and MAYBE bugfixes, not sure on their version lifecycle). Major version upgrades (like 4 -> 5) cost money, although there is no yearly fee to keep your license active (although there IS a fee if you want to keep phone/ticket support active, you only get 30 days of it free with a new license purchase).
xenForo has a lot of very solid addons, quite a few of which paid but of all the paid addons I've purchased so far, they've been incredibly quality products. I find xenForo to be much faster and I also like the default skin a lot more, as well as quite a few of the custom ones. As long as you have an active license (yearly renewal fee) you have access to all current and future versions of xenForo (even major version upgrades) as well as ticket support.
Both have large communities, I find xenForo's to be much more helpful and friendly (consider this: vB's licensed customer feedback forums aren't visible unless you're a customer -- try to guess why). As far as update frequency is concerned, that's really not a metric you can use to accurately compare things. People update software when there are actually enough things done that merit releasing a new version for, this is the culmination of months of development usually on various aspects and as such it depends on the size of the development team and what actually goes into each update. Additionally, while a small board might be fine with updating immediately every time one is shipped, updating larger boards tends to not be so easy, as the update may break custom addons/modifications, it requires possibly scheduling downtime, etc. so having them too frequent would just result in boards skipping updates. For security fixes (as opposed to feature/bugfix updates), those tend to be shipped very quickly no matter who is concerned and usually take less work to apply.
