Multi Session and/or Private
Only old Opera and Firefox (with Multifox and Private Tab) can do this well actually. Chrome has similar (Multilogin and Incognito this Tab - extensions) but not so good. It could be a great idea having this option as a part of the Vivaldi development. Any other browser has this function by default. So my vote for Multi-session tab and/or Private browsing tab.
This. Multi Login currently doesn't work with Vivaldi.
Please implement this feature multi session per tab or at least for the moment make multi login compatible. I really hope you will implement features like multifox you would really be the first browser to do so in the world.