That’s cool, but YouTube detects Vivaldi’s built in adblocker, so it’s kinda irrelevant if it’s affected by extension policies.
To use YT in Vivaldi, you have to properly configure uBlock Origin (avoid extra filters that interfere with YT) and disable the builtin adblock for YT. And given that Vivaldi relies on Chrome Extension Store for its extensions, there will still be some friction to getting Mv2 extensions after Google pulls the plug on them.