With Version 1.3, Vivaldi technologies has provided WebRTC leak protection in Vivaldi browser, to enable this feature, you need to visit settings > Privacy and uncheck ‘Broadcast IP for Best WebRTC Performance’ under WebRTC IP Handling.

But, you can’t do that in recent snapshot and stable versions of Vivaldi, because of that setting was blocked by Ublock origin extension, which can also prevents prevents WebRTC from leaking your IP address.

Can’t uncheck ‘WebRTC IP Handling’ Setting in Vivaldi browser

If you’ve Ublock Origin or any other extension installed in Vivaldi browser that prevents WebRTC from broadcasting your IP, disable it, so that you can able to uncheck WebRTC option offered in Privacy Settings. You need to wait for the extension developer to post an update on the Chrome Web Store to use the extension again.