One line to include in your robots.txt file...
Crawl_Delay: 10
Example:
User-Agent: *
Crawl-Delay: 10
Disallow: /forums/ajax.php
Disallow: /forums/attachment.php
... -- etc. --
Here 10 is the delay in seconds between page requests.
Both "Slurp" (The robot that is indexing for Yahoo and AltaVista) and the Microsoft robots for the MSN sites obey this parameter. Googlebot does not use this "Crawl-Delay" parameter yet, it just ignores it, but it will likely obey it in the next upcoming version.
Since vBulletin sites seem to be popular with search engines and lots of people have more aggresive bots than visitors at their site, it might be wise to slow down the robots by adding this line in your robots.txt file. This way the spiders do not bludgeon your serverload.
Now watch and after the robots visit your site and after some time, monitor your log files to see how many visitors came from search engines.
How to submit site to search engines?
It is free and very easy to submit your website to search engines! It's better to do that submission manually as the search engines ask for verification when submitting the site. Here are 3 popular search engines and the pages where you can add your URL:
Google:
Add your URL to Google
Yahoo:
Yahoo! Submit Your Site
MSN:
Live Search URL Submission
Note: It is up to each of the search engines to include you in their search results. The time for the inclusion also may vary depending on the number of websites in the waiting lists.