how to turn on RSS feeds for your forum as a whole (and for its individual forums), so others can add the feeds to their reader?

1. Admincp > vBulletin Options > vBulletin Options > External Data Provider > Enable RSS Syndication = YES

2. Be sure you've set Guest permissions to view forums, threads and posts. You'll get a blank page in your browser if you don't.

3. Look here and here if you'd like to add a mod that will add RSS feed icons to your forum home page.

Example :

Main forum:

Individual forums:

You can also use &lastpost=1 to show the latest posts.