How can I display the vocabulary description?
An attachment is the cleanest way to do this using the Views Admin UI. Attachments can be re-ordered in the region they are placed in the parent view.

Another way to do this which requires code is to use hook_views_pre_view() and to create a blank Views Header Area in your views admin area -- then in code via hook_views_pre_view() set the content of that area using PHP to the description of your taxonomy term. But I would recommend trying to stick to the Views Admin for creating your views, as the more custom code you put in files/modules the harder your site is to maintain or change.