if empty($TMUX)
let &t_SI = "\<Esc>]50;CursorShape=1\x7"
let &t_EI = "\<Esc>]50;CursorShape=0\x7"
let &t_SR = "\<Esc>]50;CursorShape=2\x7"
else
let &t_SI = "\<Esc>Ptmux;\<Esc>\<Esc>]50;CursorShape=1\x7\<Esc>\\"
let &t_EI = "\<Esc>Ptmux;\<Esc>\<Esc>]50;CursorShape=0\x7\<Esc>\\"
let &t_SR = "\<Esc>Ptmux;\<Esc>\<Esc>]50;CursorShape=2\x7\<Esc>\\"
endif
https://github.com/mhinz/vim-galore#change-cursor-style-dependent-on-mode
In particular, this would be cool for replace mode...