VBulletin is pretty mysql intensive, but there's also a truckload of php.
If you want it to run properly, you will need RAM, and LOTS of it. This server is a dual xeon and with 1 GB ram it would occasionally cough. Phys mem usage was 100% almost, and sometimes MySQL would go bananas (hundreds of connections, etc...) even though swap usage was almost NIL. We dropped in an extra gig, and now phys mem usage is about 1.5 gigs, but the server has not hiccuped since. The conclusion is that if your DB is very large (ours is in the gigs) then disk access is what kills the system, and mysql knows that, and tries to save you by loading everything into memory.
The PHP accelerator I installed does not cache the output of the scripts. It caches the "compiled" version of the script. The script still needs to execute, but the parsing and compiling phases are skipped. This is useful for VB since there's quite a few levels of includes in any given file. The installation is trivial, if you're familiar with building and installing unix software manually.