I finally found a solution:
Indirectly, it does have to do with my new OS. LMDE 6 is no longer based on Ubuntu, but on Debian. The Ubuntu fonts no longer exist in it, which then changed the Vivaldi UI typeface. Now I have both reinstalled the Ubuntu fonts and set Ubuntuas font-family in Vivaldi's common.css for #browser.linux,. Before it was Cantarell.
I hope that the next Vivaldi update, which will certainly overwrite this setting, will use Ubuntu fonts instead of the Sans variants. Because U. fonts are now installed and I have also selected them in the settings.
I have not yet fully figured out why Chromium and Opera show a better typeface even without Ubuntu. The default settings in Chromium, however, are Arial and Times New Roman, not DejaVu or Open Sans. Maybe that is the reason.
Update:
I couldn't help to reset the font-family to Cantarell.
And surprise:
The clearness of typeface remains still there.
That of my mind not yet, as I don't know really how it went wrong and then right again.
Anyway: I'm content to have it as desired.
Offtopic: There are more spiritual desires that aren't yet fulfilled, and the font-issue was another distraction... 😉