WARNING! If you change your user vanity name once you have used it, you will block all the wumbers issued with your old user vanity name. The wumbers generated will still be valid for each of the users that you have given them to, but you will need to re-email them your new vanity name to reactivate the existing wumbers.
Complete this process to request a user vanity name to replace your wumber number. Make sure that you understand the user vanity name request process, you can read more about it from this help page.
If you are requesting a user vanity name and you are relying on a domain name to support your application make sure that you have access to the domain name DNS as you will need to create a TXT Record and have that DNS entry verified within 48 hours.