Yep, query caching is only available in MySQL 4.x, I'd recommend upgrading to that for starters I'd also recommend installing MMCache, Erwin wrote a tutorial over at vB.com for it.
Vbulletin seems to be working fine. Right now I'm running 2.3.0 and its a bit hacked but they're quite "standard" hacks, like quick reply, pm popup box, vbarcade and member photo.
Server is a dedicated dual Xeon with 1Gb Ram. Im running latest php and mysql versions.
I've seen traffic of an average of 500 simultanous users and microstats keep displaying a server load average of 0.85...
However, sometimes, all of a sudden, something makes it go mad and server reaches loads of 170.x and up, bringing apache and mysql down...
HOW can I identify the problem? First I thought it was vbportal, but i replaced it with vbhome lite and problem persists...