If you use Vim within tmux (on Mac OSX) the clipboard can become 'detached'. To get it working again I use Chris Johnsen's excellent little command.

https://github.com/ChrisJohnsen/tmux-MacOSX-pasteboard

You can install using 'brew install reattach-to-user-namespace'

You need to config tmux:- set-option -g default-command "reattach-to-user-namespace -l zsh"

then set vim to use to use the unnamed clipboard by default.