Created many different RSS-readers for android, and simple, with a minimum of functions, and complex, with a lot of settings. But I personally (so far!) can't imagine an RSS-reader built into android/Vivaldi. Actually, where and how, from a practical point of view, do you want to embed it?
What's it supposed to look like?
The thing is that even a simple RSS-reader needs a bunch of buttons and settings. How to place it in the browser is the question!
Right now, there is no free space in the Vivaldi interface, it seems to me.
Try to draw a mockup of how it could look like. đ
And if you make the reader very simple, such as now the "reading mode" tab and the "web page source code" tab, then - will you be satisfied with this simplicity?