When the library project is built, the VectorDrawable creates PNG files for each density and places the original VectorDrawable in drawable-anydpi-v21.
If you place your VectorDrawable in drawable-anydpi-v21 in your app, then it will override the drawable from your library project. This seems like a bug and a new issue should be created (if one doesn't already exist).
NOTE: this will not replace the generated PNG files from the library. You will need to add those to your app as well to override them.