I personally don't know any high traffic forums that DON'T use vBulletin. It's the standard because it's flexible and can take punishment.
I remember back in the day when many sites were using UltimateBB and phpBB, and it was nothing but headache after headache. Loads of downtime, big database issues like rearranging entire tables when one post is deleted, forum slowdown during traffic spikes... you name it really.
vBulletin seems to have shown itself to be the best which is why everyone uses it these days
I too have been suprised in the past at the amount of issues a high traffic site can experience just from the sheer volume of people posting.