One thing should be plainly obvious. xenforo is MUCH more likely to meet the needs of a large site than vBulletin 4 !
Truthfully, I think super large sites, if not optomized, can spend $500 extra a month on bandwidth if not careful.
I'm not sure xenforo is super crazy tweaked to meet the extremely niche "Super Large site" market.
I'm sure xenforo will eventually... there are threads discussing this BTW.
Since the cost of xenforo $140 is negligible to a big site, I would expect them to probably just buy xenforo and test things behind the scenes.