mv /mnt/vps/nix/* /nix/

Clone the impermanence module repository into /etc/nixos. This requires git, if you don't have it in your system, you can use nix-shell -p git.

cd /etc/nixos
git clone https://github.com/nix-community/impermanence.git

/etc/nixos/configuration.nix

{ config, ... }:
{
  imports = [
    ./impermanence/nixos.nix
  ];
 
  environment.persistence."/persistent" = {
    hideMounts = true;
    directories = [
      "/etc/nixos"
      "/var/log"
      "/var/lib/nixos"
    ];
    files = [
      "/etc/machine-id"
    ];
  };
}