How can google indexed my vbulletin forum post/thread link within short time?

[Xtrigit]

1. Go to google's webmaster tools and submit your site.
2. create sitemap.
( you can edit the sitemap file to change the frequency etc. I set it to "hourly" (default may be yearly, monthly, daily, depending on when was the last time the thread was modified)
3. Submit sitemap
4. Change how often you want google to crawl your site (set this to the lowest value).
5. Edit/add robots.txt file, disallow any useless section.
I only allow showthread.php

That's it!