… selection absolutely does work with a vertically split vim? If the desired selection is large and spans multiple lines, yeah, you're going to have a bad time

… selection absolutely does work with a vertically split vim? If the desired selection is large and spans multiple lines, yeah, you're going to have a bad time doing it via the terminal emulator (but that has little to do with the split, other than splits make it more likely to wrap), but you can then just fall back asking vim to copy it to the selection. (This does get back at the complaint in TFA, though, that they're not the same, the moment we have to switch from the terminal to vim's command.) (All of this probably applies directly to tmux too, but I don't use tmux.)