The following setup is pretty custom, but works pretty good for me.
The only caveat is that it requires a version of Vim compiled with ruby support, so you need to install it through Homebrew:
brew install macvim --override-system-vim
Next, you need to install a few plugins:
Vimux gives you the bridge between vim and tmux, ruby-test and cucumber awareness for the test you're about to run, turbux integrates with Rails.vim to run specs from implementation.