Reason: Joomla always reserves places for all published modules no matter if it's empty or not.
To make sure position is created only when in Virtuemart - do the following:

at your template's index.php first get the values:

<?php $option = JRequest::getString('option', null); ?>

Then do a check if it's Virtuemart (com_virtuemart) and only then publish module into 'display_chp' position. (First add this position to templateDetails.xml)

<?php if ($option == 'com_virtuemart') { ?>
   <jdoc:include type="modules" name="display_chp"/>
<?php } ?>

There is also a similar question: "I have disabled ChP option to display on Product Details page, but the placeholder is still in there, so how do I remove it?".

We'll use the same approach, but this time we'll check Vrituemart's page (or view for VM2) parameter.

Virtuemart 1.1
Get current page:

<?php $page = JRequest::getString('page', null); ?>

And then tell Joomla to not create the position at all when we're on shop.product_details page:

<?php if ($page != 'shop.product_details') { ?>
   <jdoc:include type="modules" name="display_chp"/>
<?php } ?>

Virtuemart 2
Get current view:

<?php $view = JRequest::getString('view', null); ?>

And then tell Joomla to not create the position at all when we're on view=productdetails page:

<?php if ($view != 'productdetails') { ?>
   <jdoc:include type="modules" name="display_chp"/>
<?php } ?>