This is a vim omnicompletion which captures completions from zsh's completion engine. This is an application of this zsh completion capturing method.
The script works reasonably well for most cases I tested.
While it does work reasonably well, the way completion results are gathered from zsh is a HUGE HACK, so don't be too surprised with inexplicable behavior.
Install using vundle:
Bundle 'Valodim/vim-zsh-completion'
BundleInstall
The script is set as omnicompletion (^X^O) for zsh files automatically. It plays well with YouCompleteMe from what I have seen, although it needs to be manually triggered like most semantic completions.
You can also bind it to user completion (^X^U) for use outside of zsh script files using:
:set completefunc=zsh_completion#Complete