Jesse, thank you for this amazing article. I feel you. Truly describes my struggle through setting up italics in the iTerm2 + tmux + Vim setup.
For the sake of future tmux-vim land trespassers I’d like to mention that just blindly copying following statements to your`.vimrc` won’t work:

set t_ZH=^[[3m

set t_ZR=^[[23m

The “^[“ are not 2 separate characters, this is a special character that must be entered using Ctrl+V, Esc key combination.