Closed source is a boolean. Chrome is distributed only as a binary. I.e. it is totally closed source.
It currently has a very large relation to the open source Chromium project. But Google could change that tomorrow if they wanted to - they could also gradually move more and more to their closed source Chrome builds (as they have done with Android).
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.
It currently has a very large relation to the open source Chromium project. But Google could change that tomorrow if they wanted to - they could also gradually move more and more to their closed source Chrome builds (as they have done with Android).