upd

info.nix

@@ -3,5 +3,6 @@
   username = "biscuit";
   timezone = "America/Chicago";
 
+  # VPN
   vpn_enable = true;
 }

modules/system.nix

@@ -1,5 +1,10 @@
-{vpn_enable, ...}: {
+{
-  imports = [
+  vpn_enable,
+  lib,
+  ...
+}: {
+  imports =
+    [
       ../system/bootloader.nix
       ../system/firewall.nix
       ../system/locale.nix
@@ -13,6 +18,8 @@
       # ../system/touchpad.nix
       ../system/users.nix
       ../system/tools.nix
-    (if vpn_enable then ../system/vpn.nix else null)
+    ]
+    ++ lib.optionals (vpn_enable == true) [
+      ../system/vpn.nix
     ];
 }