If its the apache or mysql process causing the high load. Then here are my few suggestions on it:
- Try a different my.cnf mysql config file. You can ask Eva2000 for server optimization guidelines on the vBulletin.com forums.
- Install XCache 1.2.1, php 5.2.5.
- Try disabling the plugin system for a few minutes to check if the problem relies on some custom product.
- Set each forum's prune date to equal or less than a week. "Default View Age" option. Helps a lot if you have many threads and millions of posts.
- As a last resource, i would switch from Apache to latest Lighttpd.