Greetings!
I always use this website to check the privacy of the software and I discussed the privacy issue with a CM of Vivaldi and he sent me this official response from Vivaldi Common questions | Vivaldi Browser
They assume Vivaldi is private, but I think you need third party testing, it’s like believing Nvidia/AMD with their benchmarks. I still think Vivaldi lacks privacy if privacytest is not enough proof what should it be?
Ty
And how does one know if Vivaldi is private, we can only take the company’s word for it, can’t we do external tests?
You can, it just might not be the full picture.
I don’t know how to do these tests 
My understanding is privacytests.org is run a knowledgeable individual with many years of experience.
They have a specific page regarding Vivaldi.
Thank you very much for sharing this privacytest.org manifesto. I still believe that third party testing is important and not to blindly believe in companies.
, because “privacy” is an industry, and there a lot of ventures which rely on aggressive marketing.
It’s a solid product, but it’s created by Brave/a Brave employee. So keep that in the back of your mind.