Start a terminal and switch to a clean (empty) directory, e.g.
mkdir vivaldi-test
cd vivaldi-test
Fetch the Vivaldi rpm for the last snapshot, uncompressing it on the fly into your empty directory (e.g. 'vivaldi-test'):
wget -O- --start-pos=149904 https://downloads.vivaldi.com/snapshot/vivaldi-snapshot-4.2.2406.4-1.x86_64.rpm | xzcat | cpio -id ./opt/\*
(triple click the line above to select the entire thing. Then copy and paste it into the terminal)
Fetch a known good libffmpeg.so, uncompressing it and placing inside the extracted Vivaldi directory
wget -O- --start-pos=1152 https://launchpadlibrarian.net/555313281/chromium-codecs-ffmpeg-extra_93.0.4577.58-0ubuntu0.18.04.1_amd64.deb | tar JxO ./usr/lib/chromium-browser/libffmpeg.so > opt/vivaldi-snapshot/libffmpeg.so.4.2
Launch this test version (instead of your normal copy) and see if the issues go away
opt/vivaldi-snapshot/vivaldi-snapshot &