vim_configurable.customize {
  vimrcConfig.customRC = ''
    set number
  '';
}

####
####

error: syntax error, unexpected '{', expecting '.' or '=', at /etc/nixos/configuration.nix:16:30
(use '--show-trace' to show detailed location information)
building Nix...
error: syntax error, unexpected '{', expecting '.' or '=', at /etc/nixos/configuration.nix:16:30
(use '--show-trace' to show detailed location information)
building the system configuration...
error: syntax error, unexpected '{', expecting '.' or '=', at /etc/nixos/configuration.nix:16:30
(use '--show-trace' to show detailed location information)