I have created a greek dictionary, in UTF-8, for Vim and I have uploaded it to Github. You can find it here: https://github.com/bserem/vim-greek-spell
It is compatible with Vundle, to automatically install and update it. Checkout Vundle for more information on how to do that, or check my existing Vim configuration here: https://github.com/bserem/vimfiles/blob/master/.vimrc