I have been dealing with vBulletin for almost a year now. I manage a vBulletin installation for my friend at Fashionese Daily forum. Anyway, Fashionese Daily forum have used other forum/discusion engines like punBB and MyBB. It’s now using vBulletin 3.7.3 Patch Level 1.
It’s a pretty busy-traffic forum anyway. In average, there are 1,000 new posts everyday. According to forum stats, there are 500 – 600 online users during the prime ours (registered and guests). From the very beginning, I try to maintain its performance so that the forum is available.
Based on many articles about vBulletin optimization, here are what I did:

• Move avatars, profile pictures, attachments and CSS to file system. Make them stored as file, not in database. From vBulletin admin control panel.
  • Avatar and profile pictures settings — Go to: Avatars – Avatar Storage Type. This option was taken because there is no problem with the disk space storage.
  • Attachments — Go to: Attachments – Attachment Storage Type.
  • CSS — Go to: vBulletin Options – vBulletin Options – Style and Language Options.
  • Do not forget to check the directory permission in each path settings.
• vBulletin has some built-in optimization settings, here are few things to consider (under vBulletion Options – Server Settings and Optimization Options):
  • Update Thread Views Immediately: No
  • Duplicate Search Index Information on Thread Copy?: Yes
  • Use Remote YUI: Yes.
• Under “Cookies and HTTP Header Options” (in vBulletin Options)
  • GZIP HTML Output: Yes
  • GZIP Compression Level: 1
  • Add No-Cache HTTP Headers: No

There are still many options, if you search using Google — for example, you’ll find many setting suggestions. All settings depend on your server or preferences.
Other than that, there are also some actions taken. Again, this is based on the personal/community preferences:

• Limit unnecessary queries. For example, I just made the “Current Active users” statistics only availble for Administrators and Moderators.
• Avatars/user pictuer profiles only visible for logged in members. The same rule for signature.
• Increase time between searches. This setting can be found under “Message Searching Options”. Also, forum search is not availble for guests.

So far, the forum performance is just great. But of course, that only some methods. Every forum has its own best settings. :)