Ok I'm replying months later but better late than never I guess:
Try this:
Plugin Based Template Cache - vBulletin.org Forum
You could also try using something like PHP Accelerator.
Also you could look at installing memcache - vBulletin supports it out of the box and could give a nice little speed boost and help reduce server load.
I've also recently started serving my static content (images, javascript and css files) from a lighttpd installation instead of Apache. A basic lighty installation can serve up static way faster than apache, has a small memory footprint and will take a nice load off Apache, leaving it with more CPU cycles to handle the more intensive PHP parsing.
If it's this forum you're optimising for, I doubt you need to look at either memcache or lighty. It's only really for bigger boards - my traffic can spike to nearly 2000 simultaneous users so every little extra bit of juice I can squeeze out of the server helps.
vBulletin Community Forum - Powered by vBulletin is the best place to look for optimisation tips.