Easiest Way to Find Which Module and Layout File Assigns a Content Block to a Page

First, your layout file should be placed in app/design/frontend/{interface}/{theme}/layout/.
Second. If you want to add the css and js files only in the grouped and simple product pages don't use the <default> layout handle.
Make your layout look like this:

<?xml version="1.0"?>
<layout>
    <my_handle><!-- declare a custom handle so you won't duplicate the code -->
        <reference name="head">
            <action method="addJs">
                <script>vendor/qtip/jquery.qtip.min.js</script>
            </action>
            <action method="addCss">
                <stylesheet>vendor/qtip/jquery.qtip.min.css</stylesheet>
            </action>
        </reference>
    </my_handle>
    <PRODUCT_TYPE_simple><!-- layout handle for simple products -->
        <update handle="my_handle" /> <!-- include the handle you declared above -->
    </PRODUCT_TYPE_simple>
    <PRODUCT_TYPE_grouped><!-- layout handle for grouped products -->
        <update handle="my_handle" /> <!-- include the handle you declared above -->
    </PRODUCT_TYPE_grouped>
</layout>

Here are some sanity checks for when, like now, there are no error messages to go on.

• Are all files readable by the web server/PHP?
  I have a quick publishing script which recursively forces some file permissions. The only argument it needs is the folder to check.

  #!/bin/bash
  
  chown -R :http $* 2>/dev/null
  chmod -R g+rw $* 2>/dev/null
  find $* -type d -exec chmod g+x {} \; 2>/dev/null
  

• Is the module definitely loaded?
  At the very least it should be listed in System > Configuration > Advanced > Disable Modules Output. For a more thorough debugging see Alan Storm's Module List Module.

• Disable all caching whenever developing.
  If the old config.xml content is being used then Magento cannot be aware of new instructions to load layout files. Ditto for changes to cached layouts.

• For the handle adminhtml_bar_index the responsible action is probably in the file app/code/local/Foo/Bar/controllers/BarController.php and looks like this:

  class Foo_Bar_BarController extends Mage_Core_Controller_Front_Action {
      public function indexAction() {
          $this->loadLayout();
          $this->renderLayout();
      }
  }
  

  To check execution is getting this far put a die(__METHOD__) call in that function and refresh the page. Seeing the method name is definitive proof.

• In your config.xml you correctly have this:

      <adminhtml>
          <layout>
              <updates>
                  <bar>
                      <file>foo_bar.xml</file>
                  </bar>
              </updates>
          </layout>
      </adminhtml>
  

  Think what would happen if <bar> was also used by another module? One of the two modules would have it's contribution overruled. Try changing the name of that node to anything else.