so your getting
/usr/bin/vivaldi: error while loading shared libraries: libffmpeg.so: cannot open shared object file: no such file or directory
sudo cp /usr/lib/arm-linux-gnueabihf/libffmpeg_chrome.so.60 /lib/libffmpeg.so
While typing that line you might want to press TAB before the .60 in case your version is different?
ok, vivaldi should run now.
Regardless basically you want to copy the chromium provided libffmpeg_chrome.so.(versionnumberhere) to /lib/libffmpeg.so
Let me know if you have any problems and indeed how you fixed it if this did not work for you.