We have youcomplete and many other plugins in nixpkgs.
See https://github.com/NixOS/nixpkgs/blob/98686d3b7f9bf156331b88c2a543da224a63e0e2/pkgs/misc/vim-plugins/overrides.nix and https://github.com/NixOS/nixpkgs/blob/master/pkgs/misc/vim-plugins/generated.nix and https://github.com/NixOS/nixpkgs/blob/master/pkgs/misc/vim-plugins/overrides.nix
I use ncm2 in neovim, but we have ycmd and youcompleteme precompiled as well.
Here is how I manage mine: https://github.com/Mic92/dotfiles/blob/6c9a8461ac0cea223f7a33bdd80af8127ff304a5/nixpkgs-config/config.nix#L50
This adds a myvim package for the user that can be installed with home-manager or nix-env (nix-env -f '<nixpkgs>' -iA myvim)
You could still keep the vundle configuration in your vimrc but not running :PluginInstall on NixOS.