URLs for Bug Reports and Feedback
I think that is a great idea if the browser testers can type the URIs "vivaldi://bugreport" & "vivaldi://feedback" and report some bug or proposal. Into this, they can add a screenshot of any opened tab, report comment/bug and send to Vivaldi.
Feature Requests can also be submitted to the Bug-tracker, though I suggest searching here first.
Someone already requested adding Report a Bug on the Main Menu, which would make it easier for users who don't visit these forums often or at all.
If you search the Browser forums for previous reports of any bug that you encounter, you may find a solution, or learn that the bug is already well known.
The Vivaldi Team encourages bug reports and feedback, both here and via the Bug Tracker.