Vivaldi contains Chromium, but it isn't itself open-source, by the way.
They say of themselves that "for all practical purposes the Vivaldi source code is available for audit". I would not fully agree with that either, but I guess, at that point the open-source purists have already lost interest anyways.
There is, but on iPhone at least it sucks. I love Vivaldi on desktop - every time I try something else I quickly give up. But on mobile I can’t endorse it at the moment.
Perhaps it’s better on Android though, I don’t know.
I tried Vivaldi, it's a good browser but I prefer Brave because it has build it Tor. In my country most torrent sites are blocked so a built-in Tor is useful to me, it can open those sites without VPN.
Brave is also a shifty shady browser that has problems with inserting affiliate links without telling you and selling off user data. They're really not better or remotely trustworthy TBH, you might as well use the actual TOR browser built on Firefox if you need that capability.
They definitely did some marketing, because it came out of nowhere. When I first installed it, it was all over the internet, from YouTube to webpages. A similar thing you can notice with the Arc Browser. I couldn't find any exceptional features on the Arc Browser but the hype is encouraging people to try it.