Vivaldi has got Reader Mode feature with version 1.5, and user can be allowed to assign any keyboard shortcut for the same and use whenever he comes across articles with long text in the browser. Vivaldi team has now set a new default keyboard Shortcut for Reader View, which is Alt+R. You can change this and create another one to your liking by visiting ‘Page’ section in the Keyboard Shortcuts of Settings.
ICYMI, recently, the feature has received configuration panel, “which allows to change font type, font size, column width, line height and style of the page to light, dark or theme mode”.
For any keyboard shortcut, including this one, you can able to restore default shortcut, if modified, or delete the existing one. The latest 1.11 snapshot version has this change, will be available to the Public when Vivaldi ships stable version.
To modify or see the Reader View shortcut, click on Vivaldi menu > Settings > Keyboard, expand Page section, and check ‘Toggle Reader View’ field.