Vivaldi is compatible with most of Chrome's extensions.
These can be installed direct on Chrome Web Store.
To see which extensions are installed/enabled search for
vivaldi://extensions or just
See also Wikipedia:Google Chrome Extension.
For additional support concerning audio and video playback, install corresponding package for your version of choice:AUR, AUR.
You may installto play flash media.
Unable to set Vivaldi as default web browser
You have to edit default applications manually. Vivaldi is not able to recognize, how to do it by itself.