It could be set that “Forums are Forums.” They all work in a somewhat similar manner. Having used EE’s Forums since the introduction in 2005, and vBulletin since about the same time, I can give you a basic view of both. vBulletin has more bells and whistles; those little features that keep updates coming every so often, but that most users never figure out. EE’s Forums, while somewhat basic in the feature comparison list, provides comparable functionality, though the most important features is full integration into EE (including, as an option, the ability to run the EE Forums through EE’s template rendering engine).
So, vBulletin is very capable with many features. EE’s Forums is capable, with fewer features. Outside of that, bandwidth and server CPU will tend to be about the same (unless EE Forums runs through the template rendering engine). EE Forums does not allow for easy site design “skinning” as you will find with vBulletin, which has a whole cottage industry built to handle site themes.
A few questions you need to ask:
What do you gain by using EE and EE Forums (need both) vs. vBulletin? What do you lose (what do users lose) by switching to EE’s Forums?
Other questions: How many members would be impacted by a change? How easy will the change be for the administrator (vBulletin’s admin is far more complex than EE’s Forums). Is the site ad supported (running EE’s Forums in the template rendering engine makes it a breeze to drop in ads vs. vBulletin)?
I don’t see bandwidth as being much of an issue. Even PairLite, notoriously dependable, gives 50 gigs for as low as $8.25 a month; many other hosts are similar or even less expensive per month for comparable bandwidth.