FWIW, this is also quite possible with Vundle. I keep my plugin management in a separate file which also contains the Vundle bootstrap logic for new systems. This stanza sets up Vundle if it isn't already installed...:

https://github.com/jwhitley/vimrc/blob/master/.vim/bootstrap...

... and this one after the plugin definitions installs the remaining bundles:

https://github.com/jwhitley/vimrc/blob/master/.vim/bootstrap...

(I haven't updated this to the new "Plugin" Vundle syntax yet; the change is local but not pushed.)

That said, features like vim-plug's postupdate hooks are sweet.