This (/usr/bin/vivaldi-stable %U --password-store=basic) modified startup script with additional command line argument or whatchamacallit* seems to work: auto-login on, no pop-ups when opening vivaldi. - *I'm not programming literate.
Thank you again @ReillyBrogan for an excellent answer and detailed and responsible instructions!
@BloodFeastMan Yes, good example is always helpful! Thank you very much.
For others, who may also bump their heads at this, I also found:
https://chromium.googlesource.com/chromium/src/+/HEAD/docs/linux/password_storage.md
If I'm not mistaken, this functionality is same with chromium based browsers, which Vivaldi also is.