Vivaldi is compatible with most of Chrome's extensions.
These can be installed directly from the Chrome Web Store.
To see which extensions are installed/enabled, type
vivaldi://extensions in the address bar.
See also Wikipedia:Google Chrome Extension.
To enable proprietary media (H264, AAC, etc.) playback support:
- start Vivaldi fresh (after update or fresh install), close Vivaldi and restart again
- use the AUR from AUR
If you still need flash you can installAUR.