It's a little disingenuous so say that "Vivaldi is closed source" and leave it at that. The vast majority of their browser is built on open-source code:
Only a small portion (~5%) of their bespoke UI code is closed. The vast majority of their source is published.
Obviously whether the small portion of the code they withhold is important to you is a matter of your own to decide, but I feel this was important to clarify.
Well that part is essentially just chromium so that's why 'vivaldi' (as in what makes slightly different being its ui) is essentially closed source. But yeah that distinction is there.