Just remove the repo for your system:
sudo rm /etc/apt/sources.list.d/vivaldi.list
I hope this help you
Could you please try:
sudo apt-key add linux_signing_key.pub
It's possible a problem with Rufus, not with elementary OS. You could download the elementary ISO yourself here: https://elementary.io/
After that you can point to the downloaded image from Rufus. You could also use another tool like YUMI, UnetBootin or Balena Etcher.