Can't use mouse properly when running vim in tmux

There are two settings that you need to configure for this to work.

1. In your .vimrc add:

   set ttymouse=xterm2
   set mouse=a
   

2. In your .tmux.conf add:

   set -g mouse on
   

You will then be able to use the mouse to select blocks of text, resize splitted windows, ...

The answer by @jasonwryan won't work as of tmux 2.1 since the several mouse options were merged into one configuration called mouse.

The new approach is:

1. In your .vimrc add:

   set ttymouse=xterm2
   set mouse=a
   

2. In your tmux.conf add:

   set -g mouse on