Revert "Seems the right way to make vim's backgroup work in tmux is just add "set term=screen-256color" to .vimrc"

This reverts commit 928f7ef191716da924a7e44d730ebdfa8871d25a.

Ooops, I'm wrong.
You need add these things to make the background of vim works properly in tmux
.tmux.conf            set -g default-terminal "screen-256color"
.vimrc                set term=screen-256color
.zshrc                export TERM=xterm-256color