Vanity URL routing for Craft CMS

_vanity_router.twig

{# Treat the slug as a template path #}
{% set template = include(top_level_slug, ignore_missing = true) %}

{# Render the template contents if found, otherwise treat the slug as a username #}
{% if template is not empty %}
  {{- template|raw -}}
{% else %}
  {%- include 'users/_profile' with { username: top_level_slug } -%}
{% endif %}

_profile.twig

{# Look for the user #}
{% set user = craft.users.username(username).one() %}

{# Abort if no user was found with that username #}
{% if not user %}
  {% exit 404 %}
{% endif %}

{# Render the user profile page #}
{# ... #}

routes.php

<?php

/**
 * Dynamic Site Routes
 */

return [
  
  // Existing route declarations
  // ...
  
  // Last rule because it matches all top-level URIs
  '<top_level_slug:{slug}>' => [ 'template' => '_vanity_router' ],
  
];

Updated for Craft 3. See Craft 2 version.