You also forget the : after "Country" and the space. You also have to move the </dt> to another place in order to have the flag show up on the right side of the country field, otherwize it will be displayed below.
PHP Code:
<!-- Wy Country Flags Start -->
<vb:if condition="$post['fieldX']"> <dt>Country: <img src="http://www.YOURFORUM ADDRESS.com/forum/images/buttons/flags/{vb:raw post.fieldX}.GIF" alt="This is {vb:raw post.username}'s Country Flag" border="" /></dt></vb:if></p>
<!-- Wy Flags End -->
That's the code I'm using to have it like in your screenshot.