Text looks beautiful on MacBook Retina displays. You can use pretty much any monospace font, anti-aliased, and it will be quite readable for coding on your laptop's screen at 220 ppi. However, if you attach something with less than half that density, for example, the 93 ppi Dell U2414H (23.8" diagonal 1920 x 1200) then you get mushy fonts again.
Below are instructions for installing a pixel-perfect bitmap font which will give you back crisp text on both the external and internal Retina display and greatly reduce eye strain.
Download and install ProggyFont TrueType Font.
" add to ~/.vim/gvimrc
set guifont=ProggyCleanTT:h16
set noantialias