Vivaldi isn't open source but you can read the source code, which is better than nothing....
There is some debate inside Vivaldi about making it open source and it would be easy enough to do. I'd guess that it probably doesn't make economic sense while it has such a small market share, but I don't know if that's true.
[0]: https://www.reddit.com/r/vivaldibrowser/comments/54kzpc/psa_...