As of VuePress v0.14.4 (16 Sep 2018), there is no official solution but a hack is available.
Instead of created a page (e.g. about.md), create a directory with index.md (e.g. about/index.md).
If you are using the development server, you must access the page with a trailing slash. http://localhost:8080/about/ works but http://localhost:8080/about will not.
For production server, it will depends on your configuration and handling of trailing slash.
I am using Google App Engine to redirect pages like /about to /about/. Refer to Google App Engine Static Website Redirect Trailing Slash.
References: