@Mijaff You can assign a shortcut to Export Notes in Settings, Keyboard, Window. (None is assigned by default).
This will export each note in markdown format, which can be opened in any appropriate text editor. From there, copy/paste back into the Vivaldi Note. If you have folders in your notes, the notes will be exported into separate folders. Make your root notes folder in a directory that will not change on upgrading Vivaldi.
Vivaldi Notes.png
I don’t see much value in building in the ability to open a note in an editor. It would be better to improve the features in the built-in Note Editor, so that you don’t need this export/import step.
One can ask for anything, but as my table above shows, there are over five thousand outstanding requests so the chance of a niche feature being implemented is remote.
Take a look at existing Requests for Note Editor