In Opera (before it got to be chrome in disguise) there was the option that the browser identified itself as another browser to websites. Is it possible to implement this again? Some sites say, they only work with special browsers (i.e. https://web.whatsapp.com/) which is a bit anooying. I know that they might not work in Vivaldi anyway, but maybe they are.
Check out for user agent extensions, then you can give your customized user agent string to mask yourself as Firefox, Opera or whatever you want
I found this one for you:
It's pretty easy to use
(If you can't see the button "add to chrome" reload the page, or click on g of g plus and then click back)