Bandwidth
 

Which forum uses less bandwidth?

VB
InvisionBoard
or phpBB?

I really need to know the facts.

There's no solid answer. It depends on what style set you use, how many posts per page/threads per page you show, forums you have, avatars, attachments, etc.

vB allows you to use gzip compression which reduces, in many cases drastically, the outputted HTML filesize. I don't know if the other packages offer that off-hand.

I would say vb.... only because phpbb killed my server in bandwidth once

That's an impossible question to answer unfortunately, for the reasons that filburt said.

Gzip can be used on vb, too

You can choose Gzip compression and use CSS with vBulletin too.