The closest vim has (to my knowledge) is Unite/Denite, although fzf is functionally similar minus a couple limitations on starting new fzf searches from the callback of another.
Like helm, fzf and Unite can be fed arbitrary completion candidates, custom callbacks, mostly customized fuzzy matching. fzf is very fast, which is the main claim over what seems to be the more featureful Unite/Denite.
Does FZF work with a list of vim's ex commands as source (preferably including plugin defined commands)?
That functionality (helm-M-x) has been the best part of using emacs. It's always something I miss when I'm back in vim land. Really nice to just be able to discover these things without a mess of gui everywhere distracting me when I don't need it.
Like helm, fzf and Unite can be fed arbitrary completion candidates, custom callbacks, mostly customized fuzzy matching. fzf is very fast, which is the main claim over what seems to be the more featureful Unite/Denite.