[tbworld]

Set search type here:
Admin CP -> vBulletin Options -> Search Type

Change Character length:
Admin CP -> vBulletin Options -> Message Searching Options -> Search Index Minimum Word Length and Search Index Maximum Word Length

Rebuild Search Index:
Admincp->Maintenance->General Update Tools->Rebuild Search Index

The common words that are not indexed can be found here:
.../includes/searchwords.php

If you are running Fulltext search, the MySQL fulltext search has a 4 character minimum word length by default. This can only be changed on the server level. There are also a number of 'stopwords' that are considered common and not indexed.
Instructions for changing the default 4 character minimum in MySQL's full text search indexing:
http://www.vbulletin.com/forum/showp...57&postcount=7

Here's the list of MySQL stopwords that will not be indexed:
http://dev.mysql.com/doc/refman/5.0/...stopwords.html

I hope this helps