Seems like the custom field cloning, just a duplicates a record of the #__virtuemart_customs db table.
This is ignoring custom plugins using 3rd party tables to store part of their configuration/state.
I suggest triggering a plugin call, after the custom field cloning, so that 3rd party plugins can make use of it.
Suggestion:
In the file: administrator/components/com_virtuemart/models/custom.php
function createClone
After that code:
if (!$clone = $row->store()) {
vmError('createClone failed for ' . $id);
}
add:
elseif ($row->field_type == 'E') {
JPluginHelper::importPlugin('vmcustom');
$dispatcher = JDispatcher::getInstance();
$result = $dispatcher->trigger('plgVmOnClonePluginTable', array($row));
}
So far tested that in my plugins.