You'll need NeoBundle:
mkdir -p ~/.vim/bundle
git clone https://github.com/Shougo/neobundle.vim ~/.vim/bundle/neobundle.vim
Assuming you have Cabal, you are going to need to download a bunch of things:
cabal update && cabal install ghc-mod hoogle hdevtools hlint
If you want to use hoogle, you'll want to create its cache:
hoogle data
Autocompletion still needs work :(
Otherwise support is pretty good. Load up ghci in tmux and open a Haskell file in vim
In vim, type ctrl-c ctrl-l ; this will load the Haskell file into the ghci session
Now whenever you save, it will be reloaded in the ghci session; this is the most convenient way to REPL test Haskell code that I can think of (and is a copy of the emacs workflow)