CEO Quote from the above link.
"In addition, all of our UI code (included in normal packages) is written in plain, readable text. This means that all parts of Vivaldi are full audit-able and open from that perspective." So in other words. No that code is NOT open source. And he can stuck his "open from that perspective." BS up his ass. I am not gonna use stuff from people who do crap like this.
AFAIK the Wikipedia page for Vivaldi says its proprietary for a reason. And I don't think that reason is because of a windows installer. They release their code they build on top of chromium as open source BECAUSE THEY HAVE TO. But they build looks like they build other stuff that is "open" but not open source. At least this is what I have got from my quick look at it. This seems to follow a Opera and Chrome tradition. Even Chrome is not open source. Its 99% Chromium +1% Google crap on top of it. I am very opposed to this kind of stuff and for Opera and Vivaldi thats not just 1%.
Happy to get corrected of better educated on this.