Turns out this hotkey is hardcoded somewhere deep in webkit. What I had to do to disable this behavior was finding the name of menu item
(in my case it was File > Share > Email Link), and then going to System Preferences > Keyboard > Shortcuts tab, selecting App Shortcuts there and creating a new shortcut for Vivaldi named Email Link, and assigning something like Cmd+Alt+Shift+Control+F8 to it (to be sure I never accidentally press it)