As the fix is finally in and I could install third-party-deb packages with Ubuntu 16.04LTS now (see: bug report at launchpad and also see omgubuntu article), I installed the deb package via nautilus.
Oddly (in comparison to 14.04) I had to start vivaldi with the terminal to make the icon appear in the unity menu bar. Interestingly an error report showed up:
[7170:7170:0509/110111:ERROR:browser_plugin_guest.cc(441)] Attempting to require callback on nonexistent surface
No idea what this means.
