Well if you look at the productdetails view in Virtuemart
components\com_virtuemart\views\productdetails\tmpl/default.php
you'll find
Code:
if (!empty($this->product->customfieldsSorted['ontop'])) {
$this->position = 'ontop';
echo $this->loadTemplate('customfields');
}
this is how custom fields are loaded.
So it is your duty to edit the view as a template file to set the onbot custom field poition where you like on the VM productdetails view page.
You can then set as many customfield positions as you like and they will show where you set them in the page.
Also after having edited the file it is recommended you copy paste it to your joomla template
in
templates/mytemplatefolder/html/com_virtuemart/productdetails/default.php
as a template override, this file will be loaded instead of the virtuemart one
so you can then update virtuemart without loosing your edits.