I have been using vivaldi for a couple of months now, I simply love it. The browser is easily customisable. What I dislike is the slow startup time and in fact there are no syncronizations in the options, especially with the bookmarks. I have two computers, one for school and one for gaming/work at home. This creates a problem for me, and maybe soem tohers because I am loading up the bookmarks file from the vivaldi appdata onto my google drive and transfering it on my computer at home. This is to synchronize my bookmarks, but in browsers like opera and chrome the bookmarks and settings get synchronized. I would love for this to happen in vivaldi. Do it like chrome and incorporate the vivaldi account into the browser. Maybe people have said something like this before, but I wanted to bring this up for other users and the vivaldi developers.
Perhaps consider searching the forum for one of the other many preexisting threads discussing synchronisation for Vivaldi before starting yet another one.
Vivaldi could, of course, introduce a "user account" linked to the browser tomorrow. In fact you are commenting from a User Account right now. XMarks/LastPass also has a User Account that is "incorporated" into your browser, in that it can go into your browser files and update your bookmarks, passwords, etc. by itself, without user intervention, once it's set up.
So that's nice and all, but is the account actually "incorporated" into the browser? Not really. The browser just gets your permission and credentials to log in to an account on a server. And where is your synced data stored, so that synchronization from one machine to another is possible? In server farms that cover acres and acres of land, is where. So really, all that Vivaldi needs is a sync protocol (which they are working on) and a server farm. If you wanted to donate the server farm, I'm sure they'd be happy to have it.