Any of Vanilla's pages can be modified to use external HTML/PHP files and additional custom CSS files - demonstrating just how flexible Vanilla is in terms of styling, layout and presenting hybrid content from multiple data sources.
This page is actually called external.php and resides within the content/html directory - trying to view it directly will not work. All the content you're reading on this page was inserted into the WYSIWYG editor like regular pages.
These custom pages are 'hooked' into the system at runtime and are able to access all of your page and site variables (see below for link). The page's actual content can either be generated the same way as regular pages, as in this case, via the WYSIWYG editor, or by whatever method your external file relies upon (database, static, flat files etc.).
This very flexible feature allows you to mix and match database content with pre-prepared files - handy for throwing up ready-made pages in a hurry. Once they've been uploaded to the system they can also be edited right there in your browser using the built-in AJAX file editor.
This feature could also double up as a framework for building your own template system.
See Building a Template System for a full list of available page and site variables.