Then all the changes do in Vaadin plugin will become immediatelly available in the sample application.
Way 2
The second way is to release plugin into local maven repository. Before you do so, you need to set plugin version to SNAPSHOT.
grails set-version 7.3.0-SNAPSHOT
Then you reference SNAPSHOT version in the sample application. You can do the changes in Vaadin plugin and release it as a snapshot to your local maven repository.
grails maven-install
It is recommended to test SNAPSHOT version in the sample application everytime before you create a pull request.