It’s really really simple: when you play a song in Clementine (for now it’s hardcoded for that), the cover art, song name, artist, album name and its year will appear at the center of a simple black screen.
The code itself is very bad and hacky and was done in just a few days, just a bunch of shell commands directing to DBus to get the data Vimus needs to display. Feel free to suggest any changes or to make a pull request to make it simpler and more efficient.
The software is licensed under the GNU General Public License version 3, you can find the source code and the installable binaries over at the Git repository and at the Downloads repository.