Really Simple Syndication / Rich Site Summary or RDF is a site Summary, several similar forms of web syndication used by news websites and weblogs. It is good idea to provide RSS support for your forum.
Unfortunately it is turned off by default under Vbulletin software. Here is a quick way to turn it on so that your users can get updates via RSS readers for all new forum topics.
Step #1: Login as admin to your forum by visiting admincp (e.g. http://yourdmain.com/forum/admincp/index.php) > Click on vBulletin Options
Step #2: Select External Data Provider > Click on Edit settings button.
Step #3: Turn on "Enable RSS Syndication" and "Enable XML Syndication" (set to Yes)
Finally, click on Save button to save the changes.
Step #4: Now your forum is ready with RSS feed. To view RSS feed visit:
You can see an example of working RSS feed here.