Vivaldi is not made available under one unified open source license. It does contain the Chromium source code with changes made to allow the HTML/CSS/JS based UI to run. All changes to the Chromium source code are made available under a BSD license and can be read by anyone on vivaldi.com/source/. Details in this regard are explained in the the README and LICENSE files within the package.
In addition, our UI code is written in plain, accessible code for those who read HTML, CSS and JS. This means that for all practical purposes the Vivaldi source code is available for audit.
Vivaldi also contains third party code. Licenses for these parts can be found in the source package and in the installed browser by navigating to vivaldi://credits