Forgot to answer you, sorry.
From what I see, Vivaldi uses Chrome extensions. So if the extension works on Chrome and not Vivaldi, it's likely an issue on their side of the extensions implementation.

Because Vivaldi is built using the Chromium web browser project, extensions available in the Chrome Web Store can also be installed in Vivaldi. In some instances, extensions built for the Chrome browser will behave differently when installed in Vivaldi, but for the most part your favourite Chrome extensions will work just fine in Vivaldi.

I guess we fall in the 'in some instances' part =/