The corret path is vendor/modernizr as can be seen in foundation-rails gem:
https://github.com/zurb/foundation-rails/blob/master/vendor/assets/javascripts/vendor/modernizr.js
The path vendor/custom.modernizr gives the following error:
ActionController::RoutingError (No route matches [GET] "/javascripts/vendor/custom.modernizr.js"):
Make sure that Modernizr is included in the `<head>` of your page:
```ruby
-javascript_include_tag "vendor/custom.modernizr"
+javascript_include_tag "vendor/modernizr"
```
### Set Viewport Width
```html
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
-```
\ No newline at end of file
+```