As a quick fix you can add the following lines at the top of this file after the line:
JHTML::_('behavior.formvalidation');
Location : <jroot>/components/com_virtuemart/views/user/edit_address.php
Or override in the joomla template <jroot>/templates/<yourtemplate>/html/com_virtuemart/user/edit_address.php.
-223 is the id of USA, you can change it to your default country. You can look the id up either in the database or by inspecting the html of country drop down.
<script type="text/javascript">
jQuery(document).ready(function($) {
if ($('#virtuemart_country_id option:selected').attr('value')=="")
$('#virtuemart_country_id option[value="223"]').attr('selected', 'selected').change();
if ($('#shipto_virtuemart_country_id option:selected').attr('value')=="")
$('#shipto_virtuemart_country_id option[value="223"]').attr('selected', 'selected').change();
});
</script>
I can verify this works perfectly as of 2.0.18a
I inserted it into my template override file (<jroot>/templates/<yourtemplate>/html/com_virtuemart/user/edit_address.php) directly below the existing script, starting at line 71.
It could probably be put directly within the existing script tags but I'm a big proponent of "if it ain't broke, don't fix it"

- I pasted the whole thing in there, it worked, I moved on.