If the test page (HTML5 Proprietary Media on Linux) is a reliable indicator for video playback / a working ffmpeg plugin, it seems as if v7, at least v7.1.3570.47 has a working ffmpeg plugin on board.
After updating vivaldi on a rpi4 (aarch64), I checked the above mentioned test site and it indicated success, even though I haven't downloaded any new ffmpeg library file for a while. The last one I had added manually was for v6.9. To be certain, I removed all ffmpeg libraries of past versions I had manually put into the vivaldi directory and restarted vivaldi to see if the test site would still indicate a working ffmpeg library - it did and does.
It seems as if the times of having to manually download / extract a working ffmpeg.so file for the current vivaldi version on rpi are over - for now.
Thanks, vivaldi team!