I think i will install vBulletin 4 Sphinx Search when i have time. :8):
Why Replace vBulletin's Database Search?
If you are asking that question, chances are you don't need to. If the search system built into vBulletin 4 works well for you, keep using it. Generally, it's something that a forum owner should start thinking about ~500,000 posts, and arguably around 1,000,000 posts it becomes a necessity. vBulletin's default database search is based on MySQL FULLTEXT search. MySQL FULLTEXT search works, but it's not designed to scale well.