Dunno about Vivaldi. As a former fan of Opera I was keen the try it, and did when it was first released as a standard package for the Fedora Linux distro. However, I didn't like some features such as replacing the ability to set fonts and point sizes for different types of text with a text scaling control (I don't like serif fonts). So I went back to Firefox, every so often checking to see whether Vivaldi had improved yet. It never did. Other folks must have thought the same because Vivaldi has now vanished from Fedora - its not even an optional extra package now (Fedora 26).
Meanwhile I still dislike the way Firefox has gone with its steadily
dumbing down reduced configurability and have settled on PaleMoon as my usual browser.