You can try TYPO3 with some perfomance tricks. May be TYPO3 with vBulletin integration (I have done this integration job).
But all depends on your functionality:
- do all users must be registered
- do they use forum
You also need to do stress testing with professional stress testing software.
I use ipb and their portal, i know nuke sucks, it has the ability to take down a server, vbulletin is a resource hogg! Phpbb is good, but its not a cms... but there are mods for it though.. also why not test out mambo?