Vanity URLs has two purposes. The first is that puts your username when you share your bin's output, and secondly allows you to publish to your vanity homepage.
This means you can publish any bin to your vanity url on
<you>.jsbin.com for example: http://dave.jsbin.com
In addition, when you share the full output of your bins, your username is included in the URL, such as http://dave.jsbin.com/fajac/3/