Are you asked to log in in a tab or in a dialog that appears in the middle of a browser window?
Depending on your preference, you should log in to either your Vivaldi or Proton account in a tab that opens when you click on the relevant login option in the VPN menu.
If you see a small dialog overlapping your other tabs, that's a bug. Please let us know if that's the case.
@Vivaldi Was a temporary glitch it seems, works now. Thanks for getting back to me