This took way too long to figure out. Hope to spare someone else from the agony:

Step 1:
Install Vim that has clipboard support (OS X’s Vim does not have this)
brew install https://raw.github.com/adamv/homebrew-alt/master/duplicates/vim.rb

Make sure to update the vim alias to point to the new binary. Mine was at /usr/local/Cellar/vim/7.3.333/bin

Step 2:
(this is optional and will automatically put anything you yank into the system clipboard, if you don’t add this line, you’ll have to do “y+ to put contents into the system clipboard)
In your .vimrc
set clipboard=unnamed

Step 3:
brew install reattach-to-user-namespace —wrap-pbcopy-and-pbpaste

Step 4:
In your .tmux.conf:
# OS X Pasteboard
set-option -g default-command “reattach-to-user-namespace -l zsh”