This.

There's some good thoughtbot posts on the subject: http://robots.thoughtbot.com/a-tmux-crash-course http://robots.thoughtbot.com/seamlessly-navigate-vim-and-tmu... http://robots.thoughtbot.com/love-hate-tmux

Vim can already do horizontal and vertical splits. If you use tmux and fire up multiple instances of Vim, you'll lose some good things like ctrl+n autocompletion of words in other files.