@guigirl @Ornorm I just installed Circle Mouse Gestures, and you can actually do this with the help of that extension.
Ensure you have "Open Tabs in Current Tab Stack" enabled under Settings>Tabs>Tab Stack Options. This way when you open a link in a new tab in Vivaldi, it stays in the stack. Now, if you want it to open outside of the stack, that is where Circle Mouse Gestures comes in. Just hold down right click on the link you want to open outside the stack, and using the extensions' menu select "Background Tab". Not a native solution, but I just came across it by chance after reading anotther, unrelated thread, and trying out the mentioned extension, and had just read this post prior.