In order to make sure that fulltext searches are more efficient, common words won't be indexes. These are called Stop Words. You can see the default stop words here:
https://dev.mysql.com/doc/refman/5.0/en/fulltext-stopwords.html
However on your site, you might have words that are common enough to cause a problem that aren't on the list. For instance if your site is about florists, then flower might be a very common word that you want to restrict searching on. To add to the stop list, you would follow these steps
Create File: e.g. /etc/stopword.txt
* Change permission of this file, so that MySQL can read it.
* Don't put stopword file in /root, because mysql doesn't have permission to access it there.
Edit /etc/my.cnf file
Search for ft_stopword_file
Change Line ft_stopword_file=/etc/stopword.txt
Then run following command to restart MySQL.
service mysqld restart
Run these queries command for immediate effect of this action on the vBulletin search tables.
REPAIR TABLE searchcore_text QUICK;
REPAIR TABLE searchgroup_text QUICK;
You should include the default words in your stopword list for optimal performance.