The most striking thing about Vivaldi's decision to remain closed-source is its provenance. Granted, von Tetzchner does have a history of working with closed-source software, but one would think after the loss of Presto (excepting that one brief leak[1]), that any ex-Opera heads would understand the potential value of open source and community contribution.
As someone who put a lot into the old pre-12 Opera community, I will never use a closed-source browser again. This is not a commentary on quality - Opera was hands down the best browser ever built, despite being closed-source - but rather on commitment. If Vivaldi fails, all of its users will be left out in the cold. Again.
Admittedly, it is not quite so bad a situation as Opera 12. Vivaldi is mostly/largely open source[2]. It still seems like a massive missed opportunity though.
[1] https://github.com/github/dmca/blob/master/2017/2017-01-12-P...
[2] https://vivaldi.net/userblogs/entry/a-few-words-about-open-s...