A vanity URL is a URL or domain name, created to point to something to which it is related and indicated in the name of the URL, very similar to a Personalized URL. In many cases, this is done by a company to point to a specific product or advertising campaign micro site. In theory, vanity URLs are creatively linked to something making them easier to remember than a more random link.
This URL is stored up all links to your newsletter and is also used for the representation of landing pages and micro sites. For the recipient it is not obvious that the email campaign is sent over eyepin.