Vivaldi 1.5 or later versions have Chromecast support built-in, you don’t need to install any extension for casting.
Vivaldi team has added Chromecast support to latest Vivaldi snapshot 1.5.648.6 and is available to download for Windows and Linux platforms. After installing this snapshot, you can see Cast menu item on right click menu of a page, you can use it to cast a tab or the desktop to casting devices.
Note: We’ve noticed selecting cast menu item has triggered security alert from Windows Firewall saying some features of Vivaldi app have been blocked, if you face this issue, click on ‘Allow Access’ for Vivaldi to white list and you can able to to use the casting feature without any issues.
We were not able to install Google Cast extension on Vivaldi browser, which makes it easier to cast a tab from Vivaldi browser.