vim.vam: fix filetype detection

According to https://github.com/MarcWeber/vim-addon-manager/blob/master/doc/vim-addon-manager-additional-documentation.txt#L1310
`filetype on` must be called after all VAM plugins are initialized.
Otherwise vim set `conf` filetype for some files containing hash tags.

pkgs/misc/vim-plugins/vim-utils.nix

@@ -65,8 +65,8 @@ See vimHelpTags sample code below.
 
 CONTRIBUTING AND CUSTOMIZING
 ============================
-The example file pkgs/misc/vim-plugins/default.nix provides both: 
-* manually mantained plugins 
+The example file pkgs/misc/vim-plugins/default.nix provides both:
+* manually mantained plugins
 * plugins created by VAM's nix#ExportPluginsForNix implementation
 
 I highly recommend to lookup vim plugin attribute names at the [vim-pi] project
@@ -105,7 +105,7 @@ Then ":source %" it.
 
 nix#ExportPluginsForNix is provided by ./vim2nix
 
-A buffer will open containing the plugin derivation lines as well list 
+A buffer will open containing the plugin derivation lines as well list
 fitting the vimrcConfig.vam.pluginDictionaries option.
 
 Thus the most simple usage would be:
@@ -125,7 +125,7 @@ Thus the most simple usage would be:
       vimrcConfig.vam.knownPlugins = vimPlugins;
       vimrcConfig.vam.pluginDictionaries = [
           # the plugin list form ~/.vim-scripts turned into nix format added to
-          # the buffer created by the nix#ExportPluginsForNix 
+          # the buffer created by the nix#ExportPluginsForNix
       ];
     }
 
@@ -262,13 +262,14 @@ let
   in writeText "vimrc" ''
   " minimal setup, generated by NIX
   set nocompatible
-  filetype indent plugin on | syn on
 
   ${vamImpl}
   ${pathogenImpl}
   ${vundleImpl}
   ${neobundleImpl}
 
+  filetype indent plugin on | syn on
+
   ${customRC}
   '';