See if you have better luck if the profile sync daemon leaves Vivaldi alone altogether.
micro .config/psd/psd.conf
Scroll down the line that says BROWSERS= and delete Vivaldi out of the list.
Then restart the psd.service again, or better yet take a reboot if you can and test if the browser opens normally.
By the way, if moving the backup directory did not help at all you can either move it back:
mv ~/.config/vivaldi-backup.bak ~/.config/vivaldi-backup
or just delete it if it has already made a new one.