@erbanales, the sense of OpenSource is that devs can fork it or collaborate in a new product. Because of this all current browser engines are opensource, Gecko it's fork Goanna, Blink and the Fork Qt, WebKit and the grandfather of Blink and WebKit, KHtml, still used by KDE in it's Konqueror browser.
Gecko is only used by Firefox and forks, which is a minority in the Browser market, same as WebKit, only used by Apple. Chromium is currently the most advanced engine and because of this used by the vast majority of current browsers, practically all that come out new.
As said, Chromium is OpenSource and used by almost 70 existing browsers, the bad, also by the worst companies of the market, Google and Microsoft. No big brother company use the Gecko engine, because of this, it's easy to make Firefox and forks OpenSource, but in Chromium browsers this is different, Chromium itself is OpenSource, but apart of the engine, all other layers which are somewhat special, like those of Vivaldi with it's unique features and UI, will be forked by Google and Microsoft, even Opera, in their browsers in the same moment when it becomes OpenSource, which will be the end of all other Chromiums and also for Vivaldi.
Because of this, none of the existing Chromiums with some speciall features are fully OpenSource, not only Vivaldi, because it's existence depend on it.
OpenSource Chromiums are simple forks as is, with some minor modifications in the code and another Logo. There isn't a problem to remain FOSS.
Anyway with almost 100 existing browsers out there, all based in 3 engines, the sense of OpenSource is very debatable, more for the user, for him prevail more the Company etics its transaparency and a good community, which is full given in Vivaldi, despite that 5% of it's script related to it's UI is protected, but full auditable and even moddeable by the user, the only thing, he can't use it for another browser.
OpenSource is fine, but don't confuse OpenSource as synonym of security and privacy or beeing trustworth, this depends only on the author or company and what he does with the fork he use.