I would appreciate it if I can store and reinstall the registered webpage passwords (settings - privacy) in a separated file on my hard drive. When I have to install the OS usually these passwords are gone. Perhaps you can combine it with the export and import of the bookmarks (file - export bookmarks/import bookmarks and settings).
Passwords of logins are encrypted by a key bound to local user-id of the used account in the OS. If you copy to another/new-created account or reinstall OS, the encryption key is lost.
Before reinstalling OS you can workaroudn this püroblem:
Activate the hidden feature in in vivaldi://flags/#password-import-export to allow a export/reimport of passwords as CSV file.
Caution: these exported passwords are readable by others who can use you PC.